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 combines a glass-bottom view of the National Marine Park of Zakynthos with swimming near the Keri Caves and a stop on Marathonisi, the island also known as Turtle Island. I especially like the chance to see Caretta Caretta turtles in their natural setting and the friendly, multilingual captains who help keep the trip relaxed.

I also like the simple format. At $29 per person, you get the boat ride, the glass-bottom viewing, the Keri Caves area, an Oasi Bay swim stop, and time on Marathonisi without paying for a full-day cruise. The main drawback is that turtle sightings are never guaranteed, and the protected beach on Marathonisi is small. You may also find several boats gathering around a single turtle.

The boat leaves from Agios Sostis harbor, near Laganas. Look for the Nautilos flag in front of the boat, where the captain will welcome you. You can sit in the shaded area or use the rear sundeck, so bring sunscreen, a hat, sunglasses, swimwear, a towel, and a small bag rather than bulky luggage.

Key points to know before you go

• Turtle spotting is possible, not promised: The boat enters protected waters where loggerhead turtles feed and nest, but wildlife sets the schedule.
• Swimming takes place near the caves: You can swim in the Keri Caves area and at Oasi Bay, but swimming inside the cave itself is not allowed.
• Marathonisi has limited beach access: Only a small part of the shore is open because the rest is protected turtle nesting ground.
• The glass-bottom boat adds another viewpoint: You can watch the water below while moving between stops, though visibility depends on sea conditions.
• The trip lasts about three hours: That gives you a compact taste of the coast rather than a full-day cruise.
• The $29 price is strong value: Food, drinks, and hotel transport cost extra, but the main boat experience and stops are included.

Leaving Agios Sostis for the Marine Park

The trip begins at Agios Sostis harbor, close to Laganas. This is a meeting-point tour, so you need to make your own way to the harbor and locate the Nautilos flag directly in front of the boat. The captain welcomes you there, and the host can speak Greek and English.

The boat offers two useful seating choices. The shaded section is best if you want protection from the strong Greek sun, while the rear sundeck gives you more open air and a chance to work on your tan. I would choose shade for the outbound ride and move around when the boat slows for photos.

As the boat heads toward the National Marine Park of Zakynthos, the glass-bottom panels let you watch for fish and other underwater sights. This feature is most useful when the boat is moving slowly or stopped, rather than when it is cruising at speed. Still, it gives children and anyone who prefers to stay dry a way to share in the marine setting.

The boat ride itself is part of the appeal. You get views of the Zakynthos coast, open water, rocky coves, and the green interior of Marathonisi as the route continues. Several people particularly praised the captains for being friendly, funny, and accommodating. One captain was also noted for speaking several languages, which helps when a mixed international group is on board.

Swimming near the Keri Caves and Oasi Bay

The first major stop is the Keri Caves area, reached only by boat. The limestone cliffs form small inlets and openings where sunlight reflects off the water. The effect is especially striking from the sea, where the blue water meets pale rock.

You can jump into the water here for a swim, provided conditions allow. The stop is intended for swimming near the caves, not inside them. That distinction matters: the tour information specifically says that swimming inside the cave is not permitted.

A second swimming area is usually Oasi Bay, close to the Keri Caves. This gives you another chance to get into the water, take photographs, or use snorkeling equipment. Bring your own mask and snorkel if you want to look below the surface, since snorkeling gear is not listed as included.

The swimming time is a strong part of the trip, but you should take care around rocks. One person cut a foot after slipping, and although the captain helped with the situation, the incident is a useful warning. Wear water shoes if you have them, move slowly near the shoreline, and do not rush into shallow water where rocks may be difficult to see.

The water is the star here, but do not expect a long, private swim. This is a shared boat excursion, and the timing has to work for the whole group. You will get a refreshing stop and good photographs rather than an extended beach day.

Looking for loggerhead turtles from the boat

Turtle spotting is one of the main reasons people choose this cruise. Marathonisi and the surrounding waters form part of the protected habitat of the endangered loggerhead turtle, known locally as Caretta Caretta. The turtles lay their eggs on protected beaches, and the small accessible beach on Marathonisi is carefully restricted for that reason.

The boat may slow or stop when a turtle is seen. You can watch from the deck or through the glass-bottom panels, depending on the position of the animal. Seeing a turtle in its natural setting can be a wonderful moment, especially for children. Several families found this to be the highlight of the outing.

But wildlife watching needs realistic expectations. You might see one turtle, several, or none. The time of year can also affect sightings. One July trip had turtle sightings even though some turtles had already left the area as their eggs began hatching.

There is also an environmental concern. Boats can gather around one turtle, and too much chasing or crowding is not good for the animal. I would treat a sighting as a privilege, not a reason to demand a close approach. Keep your distance, follow the captain’s instructions, and enjoy whatever view the turtle allows.

This is an important point when comparing boat tours. A promise of guaranteed turtles would be misleading. The real value is access to the protected marine area, with a reasonable chance of seeing wildlife alongside the caves and island visit.

Arriving at Marathonisi, the Turtle Island

Marathonisi appears with pine, olive, and green oak trees covering its interior. From the water, it looks more natural and undeveloped than the busy resort areas around Laganas. The island’s shape and turtle connection make it one of the most recognizable stops in this part of Zakynthos.

You get free time on the island to explore and relax. A bar sells snacks, drinks, and ice cream, but these purchases are not included in the $29 price. If you want to keep costs down, bring your own food and water. The practical instructions also encourage you to bring food and drinks with you.

Beach access is limited. Only a small section is open because the rest of the shore is a loggerhead breeding area. Do not expect to wander along the entire beach or find a long stretch of open sand. The restriction is part of the point: this is a protected place, not simply another swimming beach.

Some people find Marathonisi’s beach fairly ordinary compared with the caves and open-water stops. I think that is a fair criticism. The island is valuable mainly for its protected status, scenery, and turtle connection, not because it offers a perfect beach day.

The stop can still be pleasant. You can buy an ice cream, sit in the shade, take photographs, and enjoy a short break from the boat. Several visitors appreciated having roughly an hour on the island, though the exact schedule may vary. Bring a hat and sunscreen because the available shade can matter on a hot day.

How the three-hour schedule works

The advertised duration is about three hours, which makes this easy to fit into a stay in Laganas or nearby Agios Sostis. It is long enough to include the boat ride, a turtle search, swimming near the Keri Caves, and time on Marathonisi without taking up an entire day.

The schedule is not a minute-by-minute sightseeing program. The captain may adjust the route or timing for sea conditions, wildlife, and the needs of the group. A short delay has occurred on some departures, with advance notice given, so allow a little flexibility rather than planning a tight connection afterward.

The middle portion may include a longer island break than some people expect. If you prefer constant movement, that pause could feel slow. If you want a chance to eat, rest, and dry off, it is useful.

The boat is described as roomy, with enough space for shaded seating and an open rear deck. Still, this is a shared excursion, not a private charter. Your comfort depends partly on how full the boat is and where you choose to sit.

What to bring for a comfortable trip

Pack lightly. Large bags and luggage are not allowed, so bring only what you need for several hours on the water.

You should have:

• Swimwear and a towel
• Sunscreen, sunglasses, and a sun hat
• Water and food if you do not want to buy them on Marathonisi
• Snorkeling gear if you plan to look below the surface
• A phone or camera in a waterproof case

A waterproof pouch is a sensible extra for cash, phones, and room keys, though it is not specifically listed as required. Avoid bringing anything you cannot protect from spray or wet swimsuits.

If you are prone to sunburn, use the shaded seating area. If you want the best open views, spend some time on the rear sundeck, but do not stay exposed for the entire ride.

Is $29 good value for the experience?

At $29 per person, this is a good-value introduction to the southwest coast of Zakynthos. The price includes the glass-bottom boat ticket, the visit to Marathonisi, swimming near Oasi Bay and the Keri Caves area, and the return journey to Agios Sostis.

You still need to pay for your own food, drinks, and transport to the harbor. That keeps the base price low, but it means your final cost will be higher if you buy lunch, ice cream, and beverages on the island.

Compared with booking a private boat, this is a much cheaper way to see the caves and Marine Park. The tradeoff is less control over the schedule and less privacy. You also share the turtle search with other boats, which can affect the atmosphere.

The value is strongest if you want several highlights in one short outing: wildlife, scenery, swimming, and an island stop. It is less compelling if your main goal is a quiet beach or a long, unhurried snorkel.

The experience has a rating of 4.3 from 401 reviews, which fits the picture here: consistently enjoyable, with some predictable limitations around crowds, the modest beach, and turtle-spotting behavior.

Who should book this boat trip?

I would recommend it to families, couples, and first-time visitors who want an easy introduction to the coast near Laganas. Children often enjoy the glass-bottom boat and the excitement of looking for turtles, while adults get swimming time and dramatic coastal views.

It also suits anyone with only a half-day available. You do not need to commit to a full-day sailing trip, and the three-hour length leaves time for other plans.

You may want a different outing if you dislike shared boats, want a guaranteed wildlife sighting, or hope to spend most of the day on a broad sandy beach. The Marathonisi beach is restricted, and the caves are viewed from the water rather than used as a swimming tunnel.

The captains are a major part of the experience. Their friendliness, humor, language skills, and willingness to help received repeated praise. That personal touch matters on a short trip, especially when the captain is handling swimming stops, wildlife viewing, and a mixed group.

Should you book the Agios Sostis boat tour?

Book it if you want a compact, affordable day on the water with several different stops. The best parts are the Keri Caves scenery, the clear swimming areas, the chance to see Caretta Caretta turtles, and the relaxed time on Marathonisi.

Go with sensible expectations. You may not see turtles, you cannot swim inside the caves, and the island beach is small by design. If you respect those limits, this $29 trip offers a lot in three hours and gives you a practical way to experience the Marine Park without paying for a private cruise.

The tour offers free cancellation up to 24 hours before departure, and you can reserve now and pay later. That is useful if your plans depend on weather or if you want to keep your schedule flexible.

FAQ

Where does the boat tour meet?

The meeting point is at 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 tour last?

The tour lasts about three hours. Starting times depend on availability.

Is hotel pickup included?

No. Hotel pickup and drop-off are not included, so you need to reach Agios Sostis harbor yourself.

Can I swim inside the Keri Caves?

No. You can swim in the Keri Caves area and at Oasi Bay, but swimming inside the cave is not allowed.

Is food included?

No. Food and drinks are not included. You can buy snacks, drinks, and ice cream from the bar on Marathonisi, or bring your own supplies.

Can I see turtles on every trip?

No. The boat looks for loggerhead turtles in their natural habitat, but sightings are not guaranteed. Turtle activity can also vary by season.

Is all of Marathonisi open to visitors?

No. Only a small part of the beach is accessible because the beach is protected as a turtle breeding ground.