Agios Sostis: Marathonissi, Keri Caves, and Turtle Spotting

Turtles and clear water make this a memorable morning. This three-hour boat trip from Agios Sostis takes you into Zakynthos National Marine Park, with stops near the Keri Caves, Oasi Bay, and Marathonisi, also called Turtle Island. I like the glass-bottom boat, which gives you a chance to watch the water below while moving between stops, and I like the friendly captains, who have been praised for sharing stories, handling several languages, and helping passengers feel at ease.

The $29 price is fair for a three-hour trip that includes the boat ride, a swim stop, views of the caves, and time on Marathonisi. The main consideration is that turtle sightings are never guaranteed, and several boats may gather around one animal. Also, you cannot swim inside the cave itself. The swim takes place in the surrounding Keri Caves area, usually near Oasi Bay.

Key points to know before you go

• Look for the Nautilos flag: The meeting point is in Agios Sostis harbor, directly in front of the boat.
• The swim is near the caves, not inside them: Expect a stop at Oasi Bay or the nearby Keri Caves area.
• Marathonisi has limited beach access: The turtle nesting beach is protected, so only a small part is open to people.
• The glass bottom is useful between stops: It lets you watch the sea below, though water conditions affect how much you see.
• Turtle sightings depend on nature: You may see Caretta Caretta turtles, but no boat trip can promise it.
• Bring your own basics: Pack swimwear, a towel, sunscreen, a hat, sunglasses, food, drinks, and snorkeling gear.

Setting out from Agios Sostis harbor

You meet the boat at Agios Sostis harbor in Laganas. Look for the Nautilos flag placed just in front of the boat. The captain welcomes you there, so you do not need to search through the harbor for an office or a separate departure point.

There is no hotel pickup or drop-off. That matters if you are staying away from Laganas, since you need to arrange your own way to the harbor. You also should leave large luggage behind. A small day bag with your swim gear is the sensible choice.

The boat has a shaded seating area and a rear sundeck. I would choose the shade for the outward ride if you want to save your skin for the swimming stop, then move to the rear deck when you want more sun and open views. Bring a hat and sunglasses because the reflected light on the water can be strong.

The trip lasts about three hours, which gives it a useful place in a busy Zakynthos schedule. You can fit it into a morning or afternoon without giving up an entire day. One departure experienced a slight delay, but passengers were notified beforehand and the captain worked to accommodate everyone. I would still avoid planning a tight connection immediately afterward.

The glass-bottom ride through the marine park

The main appeal of this outing is not simply getting from one beach to another. You are cruising through the protected waters of the National Marine Park of Zakynthos, an area associated with the endangered Caretta Caretta loggerhead turtle.

The glass-bottom section gives you another way to watch the sea. You may see fish or underwater shapes below the boat, though visibility can change with sunlight, waves, and water movement. It is not a private underwater viewing room, but it adds interest during the ride and works well for children who want something to watch between stops.

The boat also gives you a broad view of the coast. The Keri area is known for pale limestone formations, sea caves, and water that changes from clear green near shore to darker blue farther out. Keep your camera ready as the boat approaches the rocky coast. The views are a major part of the value, particularly if you enjoy taking pictures from the water rather than only from a beach.

Captains have been praised for being funny, kind, and accommodating. One captain was especially appreciated for speaking several languages, while another helped after a passenger slipped on a rock and cut a foot. That does not remove the need for care around rocky swimming areas, but it suggests that the crew takes practical problems seriously.

Swimming near the Keri Caves and Oasi Bay

The boat usually stops at Oasi Bay or close to the Keri Caves area. This is the part of the outing built for swimming, snorkeling, and a quick escape from the heat.

The water is described as clear and beautiful, and the rocky setting gives the stop more character than a simple beach break. You can swim in the deep blue water near the caves and look toward the limestone walls as light plays across them. The stop is also a good place for photographs, especially if you take pictures from the boat before getting wet.

Be clear about one detail: you do not swim inside the cave. The boat stops in the surrounding area, and you swim near the cave zone or at Oasi Bay. That distinction matters if you are booking because you picture yourself entering a dark cave by water.

You should bring snorkeling gear if you have it. It is listed as something to bring, not as equipment supplied with the tour. A mask and snorkel can make the stop more rewarding, but even without them, the clear water and rocky coast are the main attractions.

The swim stop has one practical drawback. Rocks can be slippery and uneven, so enter the water carefully and watch your footing when climbing back aboard. One passenger cut a foot after slipping on a rock, a useful reminder that a beautiful swimming spot is not always an easy one.

Reaching Marathonisi, the Turtle Island

After the caves, the boat continues toward Marathonisi. From the water, you can see the island’s pine, olive, and green oak-covered slopes. The approach is scenic, and the island feels more natural than a developed resort beach.

Marathonisi is important as a breeding ground for loggerhead turtles. The small sandy beach is where the turtles lay their eggs, so access is restricted. You do not get the whole beach to yourself, and you cannot wander wherever you please. Only a small portion is open for people, with the protected nesting area kept off limits.

That limited access is good for the turtles, but it shapes your experience. If you expect a long beach walk or a full afternoon of beach lounging, you may find the stop modest. One particularly useful assessment called the beach mediocre, and that is fair if you compare it with Zakynthos beaches chosen purely for sand and facilities.

I see Marathonisi differently as part of this boat trip. You are visiting a protected island connected to the marine park, not booking a luxury beach club. The value comes from the setting, the short time ashore, and the chance to understand why parts of the island are closed.

You have free time to explore the accessible area or buy something at the bar. Snacks, drinks, and ice cream are available for purchase, but they are not included in the $29 price. Bring food and drinks if you want to avoid spending extra or if you need something specific.

There is also some shade on the island, which is helpful in summer. Still, pack sun protection. A hat, sunscreen, and water are more useful than hoping to find a comfortable shaded place at the exact moment you arrive.

Looking for Caretta Caretta turtles

Turtle spotting is the most exciting part of the trip, but it is also the least predictable. You are looking for animals in their natural habitat, not visiting an aquarium with a scheduled appearance.

Several people on these trips have been lucky enough to see one or more turtles. The sight can be special, especially for children. One family described the moment as a highlight for a young boy, while another outing spotted a turtle even though the season was already moving on.

Timing matters. In July, turtles may already be leaving parts of the area as eggs hatch, so sightings can become less likely as the season progresses. That is not a rule you can use as a guarantee, but it is a useful reason to keep expectations realistic.

There is also an environmental concern. At least one outing had several boats gathering around a single turtle. I find that troubling, and you may feel the same. A good wildlife encounter should leave the animal room to move. If boats begin crowding a turtle, keep your own attention on respectful viewing rather than treating the moment like a chase.

The crew’s role matters here. Captains have been praised for locating turtles and showing passengers the sights, but the best experience depends on how the boat approaches and how long it stays. You can enjoy the sight without expecting a close-up photograph. In fact, a distant, calm view is often better for the animal.

Is three hours enough?

Three hours is enough for a sample of the area, not a full day on the water. You get the cruise, a swim stop, views of the Keri Caves, time at Marathonisi, and a chance to look for turtles.

The schedule can feel well balanced when the boat allows enough time to swim and take pictures. Several outings included a useful beach break of about an hour, although one passenger felt the length of the break had not been explained clearly in advance. I would ask the captain about the timing at departure if you want to know exactly how long you will spend at each stop.

The short format suits you if you want a relaxed half-day activity. It is also a good choice for families who may not want a long boat excursion. You get several different scenes without committing the whole day to a single route.

If you want extended snorkeling, a quiet beach day, or a slower private cruise, three hours may feel brief. The boat covers several highlights quickly, so the experience favors variety over long stays.

What the $29 price really buys

At $29 per person, this is one of the easier excursions to justify in a Zakynthos holiday budget. The ticket includes the glass-bottom boat, the Marathonisi visit, the swim stop, and the cruise past the Keri Caves and Oasi Bay area.

You are paying for access to places that are reached by boat, plus the convenience of combining them in one outing. Renting or arranging separate boat trips would likely be less simple, while a private boat would cost far more. The friendly crew and roomy boat add to the sense of value.

The price does not include hotel transport, food, drinks, or snorkeling gear. Those costs are easy to manage if you know about them ahead of time. Bring your own towel, water, snacks, and mask if you want to keep the outing close to the advertised price.

The boat has received a 4.3 rating from 401 reviews, with especially strong praise for the captains, the scenery, the swim stop, and the turtle sightings. The lower points are practical rather than surprising: Marathonisi beach may seem ordinary, turtle watching can become crowded, and some timing details may need clearer explanation.

Who will enjoy this boat trip most?

I would recommend it to you if you want a low-cost introduction to Zakynthos from the water. It works especially well for families, couples, and first-time visitors who want to combine wildlife, swimming, caves, and a beach stop without spending all day aboard.

Children may enjoy the glass bottom and the possibility of seeing turtles. Adults will likely appreciate the scenery and the chance to swim in clear water. The crew’s easygoing approach is another plus if you value a friendly, informal outing rather than a formal guided cruise.

You should think twice if your main goal is a flawless sandy beach. Marathonisi offers a protected and scenic stop, but the accessible beach area is small and may not impress you as much as the Keri coastline.

You should also avoid booking with unrealistic wildlife expectations. The turtles are wild, sightings vary by season, and several boats may be present. The caves and swimming stop remain worthwhile even if the turtles stay out of sight.

Booking is flexible, with free cancellation up to 24 hours before departure and a reserve-now, pay-later option. The host or greeter speaks Greek and English. Check the available start times, bring only a small bag, and arrive at the Nautilos flag in Agios Sostis harbor.

Should you book this Agios Sostis excursion?

Book it if you want good value and several highlights in three hours. The strongest reasons are the Keri coastline, the swim near Oasi Bay, the protected setting of Marathonisi, and the chance to see a loggerhead turtle in the wild.

I would not book it solely for a long beach stay or a guaranteed close turtle encounter. Go with flexible expectations, protect your feet around the rocks, and treat any turtle sighting as a privilege rather than a scheduled show. For $29, this is a sensible choice for a compact, scenic boat trip from Laganas.

FAQ

Where does the tour depart from?

It departs from Agios Sostis harbor in Laganas. Look for the Nautilos flag directly in front of the boat, where the captain will welcome you.

How long does the boat trip last?

The tour lasts three hours.

Is hotel pickup included?

No. Hotel pickup and drop-off are not included, so you need to make your own way to Agios Sostis harbor.

What places does the tour visit?

The trip includes the Keri Caves area, Oasi Bay for swimming, and Marathonisi, also known as Turtle Island. It also travels through the National Marine Park of Zakynthos.

Can I swim inside the Keri Caves?

No. Swimming inside the cave is not allowed. The swim stop takes place near the Keri Caves area, usually at or close to Oasi Bay.

Are turtles guaranteed?

No. Turtles live in their natural habitat, so sightings are not guaranteed. Some departures spot turtles, but seasonal conditions can affect how likely this is.

How much time do I get on Marathonisi?

You have free time on the island, and some trips include about an hour for the beach stop. Ask the captain about the exact timing when you board.

Are food and drinks included?

No. Food and drinks are available to purchase at the bar on Marathonisi, or you can bring your own.

What should I bring?

Bring sunglasses, a sun hat, swimwear, a towel, sunscreen, food and drinks, and snorkeling gear. Large bags and luggage are not allowed.