Agios Sostis: Marathonissi, Keri Caves, and Turtle Spotting

The main attraction here is the chance to see loggerhead turtles in their natural habitat while cruising through Zakynthos National Marine Park. I like the clear-water swim stop near Oasi Bay and the simple, good-value format: three hours on the sea for $29, with a glass-bottom boat and Marathonisi visit included. The main caution is that turtle sightings are never guaranteed, and several boats may gather around one animal.

I also like the choice of shaded seats or a rear sun deck, plus the easy meeting point at Agios Sostis harbor. The stop on Marathonisi is pleasant, but only a small part of its beach is open because it is a protected nesting area, and one hour there may feel long if you want a more active island visit.

Key points to know before you book

• Turtle spotting is the headline: You may see Caretta Caretta loggerhead turtles from the boat, but sightings depend on the season and the animals.
• Oasi Bay is the main swim stop: You can swim and snorkel in clear water near the Keri Caves area, but swimming inside the cave itself is not allowed.
• Marathonisi is a protected turtle island: You get beach time, but only a small section is accessible.
• The boat has useful seating choices: Pick shade for comfort or the rear sundeck for sun and open views.
• The $29 price is strong value: The three-hour cruise includes the boat ride, turtle-island visit, cave views, and a swim stop.
• Pack lightly: Large bags and luggage are not allowed, so bring only beach essentials.

A three-hour cruise from Agios Sostis

This is a short, focused boat trip rather than an all-day excursion. You meet the captain at the Nautilos flag in Agios Sostis harbor, with the boat directly in front of it. The captain welcomes you aboard, and the trip heads toward the protected waters around southern Zakynthos.

The boat has a shaded seating area and a rear sundeck. That matters in the Greek sun. You can stay out of the heat while watching the coast, or sit behind the boat with a more open view and a chance to work on your tan. Bring a hat and sunscreen either way, since the water reflects plenty of light.

The boat is described as roomy, and the three-hour length gives you enough time for the main sights without turning the day into a long commitment. A slight departure delay has happened, but the group was notified in advance and the captain worked to accommodate everyone. I would still avoid scheduling this too close to another fixed activity.

The trip is hosted in Greek and English. Several reports praise the captain’s friendly manner, humor, helpfulness, and ability to explain the sights to people speaking different languages. The atmosphere seems relaxed rather than formal, which suits a swim-and-sightseeing cruise.

Looking through the glass-bottom boat

The glass bottom gives you a view beneath the boat as you move through the national marine park. It is not the same as snorkeling, and you should not expect a full underwater tour. Its value is that you can watch the water below while staying seated, especially when the boat pauses or moves slowly near interesting areas.

The glass bottom is also useful for families. A young child may not want to spend much time in the water, but can still take part in the search for turtles and watch the sea below. Keep your expectations sensible. The main pleasure is the combination of open-water scenery, underwater glimpses, and the possibility of spotting a turtle.

Turtle sightings vary with conditions and the time of year. In July, for example, some turtles may already have moved away from the area because their eggs are hatching. Other trips have seen one turtle, or several, and the sight can be memorable when it happens.

You should also think about the ethics of turtle watching. One account described four boats gathering around a single turtle, which can feel uncomfortable. The best approach is to watch quietly, avoid encouraging the boat to crowd or chase an animal, and remember that the turtle’s welfare matters more than getting a perfect photograph.

Swimming near the Keri Caves and Oasi Bay

The boat usually stops near Oasi Bay in the Keri Caves area. This is the active part of the excursion, so bring swimwear, a towel, sunscreen, and snorkeling gear if you have it. The water is described as clear, and the swim stop gives you time to cool off rather than only viewing the coast from the deck.

The Keri Caves are reached by boat, with limestone walls and blue light reflecting across the water. The setting is best appreciated from the sea, where you can see the shapes of the rock and take photographs from a useful distance. The boat may pause for pictures before or around the swim stop.

There is an important limitation: you cannot swim inside the cave. The swim takes place in the surrounding area, often close to Oasi Bay. That is still a worthwhile stop, but anyone booking specifically to paddle deep into a cave should adjust those expectations.

The rocks and entry points can be slippery. One person cut a foot after slipping on a rock, although the boat driver helped with the situation. Use care when entering or leaving the water, and do not rush onto wet stone for a photograph.

This stop is one of the best reasons to choose the trip. You get a real break from sitting on a boat, and the clear water offers more than a quick photo stop. Bring your own mask and snorkel if you have them, since snorkeling gear is listed as something to bring rather than something included.

Marathonisi, the turtle island

After the cave area, the boat continues to Marathonisi, also known as Turtle Island. As you approach, you can see pine, olive, and green oak growth across the island. The setting feels quieter and more natural than a developed beach resort, though your time on shore is limited.

Marathonisi is a breeding ground for Caretta Caretta turtles. The small beach is protected, so only a limited section is open for people. This rule is important, not inconvenient. It protects the place where turtles lay their eggs, and it keeps the stop from becoming a normal open beach visit.

You get free time to walk around the accessible area, swim, or rest. A bar sells snacks, drinks, and ice cream, but food and drinks are not included. You can also bring your own food and water, which is sensible if you want something specific or prefer not to spend extra at the beach.

There is some shade on the island, which helps during the warmer months. Still, bring a sun hat and sunscreen. The beach stop lasts roughly an hour according to the practical details in the trip information, though the exact timing may vary.

Here is the honest tradeoff: Marathonisi is more valuable for its protected status and setting than for the beach itself. One assessment found the beach fairly ordinary. If you want dramatic scenery, turtle conservation context, and a pleasant swim, it fits the trip well. If you want a long, active island exploration, this stop may feel limited.

What the $29 price really includes

At $29 per person, this is an inexpensive way to spend three hours on the water. The price includes the glass-bottom boat, the visit to Marathonisi, a swim stop, and views of the Keri Caves. It does not include hotel transport, food, drinks, or snorkeling equipment.

That makes the value strongest for people staying near Agios Sostis or Laganas who can reach the harbor on their own. You are paying for a compact marine-park outing, not a private charter or a full-day boat adventure. The combination of turtle spotting, cave scenery, and swimming gives the fare a good balance.

The low price also explains why the experience may feel shared and simple. There is no promise of a private boat, a long guided talk, or an elaborate meal. What you get is a friendly captain, a comfortable boat, basic facilities, and several attractive stops packed into three hours.

The overall rating is 4.3 from 401 reviews, which fits the experience well. The strongest praise centers on the captains, the friendly atmosphere, the clear water, the views, and the chance to see turtles. The common reservations are the limited beach access, uncertainty around turtle sightings, occasional departure delays, and boats gathering around wildlife.

Who will enjoy this trip most

I would recommend this excursion to families, couples, and first-time visitors who want a simple introduction to Zakynthos from the water. The short duration works well when you want to leave room in the day for lunch, a beach visit, or another activity.

It is also a good pick if you care more about variety than luxury. You get a boat ride, a glass-bottom view, a marine-park setting, a swim, cave scenery, and an island stop. None of these parts lasts all day, but together they create a well-rounded outing.

Families should appreciate the shaded seating and the possibility of seeing turtles without requiring everyone to snorkel. Children who enjoy swimming will likely get the most from the Oasi Bay stop and the beach time on Marathonisi.

You may want something else if you dislike shared boat trips, need guaranteed wildlife sightings, or want a large beach with unrestricted access. It is also not ideal if you expect to swim inside the Keri Caves. That is specifically not permitted.

Practical tips for a smoother outing

Meet at the Nautilos flag in Agios Sostis harbor and look for the captain at the boat. Hotel pickup and drop-off are not included, so plan your own way to the harbor.

Pack sunglasses, a sun hat, swimwear, a towel, sunscreen, water, and any food you want during the cruise. Snorkeling gear is also worth bringing. Large bags and luggage are not allowed, so use a small beach bag rather than arriving with suitcases.

Keep your camera ready for the Keri Caves, the marine park, and the approach to Marathonisi. A waterproof phone pouch can be useful around the swim stop, though that item is not provided.

If turtle spotting is your main goal, treat it as a possibility rather than a guarantee. The animals may move away from the area, especially later in the season. If you do see one, enjoy the moment without expecting the captain to chase it for a close-up view.

The booking terms are flexible: you can cancel up to 24 hours before departure for a full refund, and reserve now while paying later. Starting times depend on availability, so check the schedule before building the rest of your day around it.

Should you book the Agios Sostis boat trip?

Book it if you want an affordable, three-hour introduction to Zakynthos by sea. The best parts are the Oasi Bay swim, the Keri Cave scenery, and the chance to see a loggerhead turtle. The friendly captains and choice of shade or sun make the trip easier for families and mixed-age groups.

Skip it if Marathonisi’s limited beach access sounds frustrating, or if you want a private, quiet wildlife encounter. You should also pass if swimming inside a cave is the central reason you are booking, since that is not allowed.

For most people staying around Laganas or Agios Sostis, $29 is a fair price for the range of stops. Just bring light beach gear, keep your turtle expectations realistic, and judge the trip as a compact marine-park cruise rather than a full island expedition.

FAQ

Where does the boat tour meet?

The meeting point is at the Agios Sostis harbor in Laganas. Look for the Nautilos flag in front of the boat, where the captain will welcome you.

How long does the tour last?

The tour lasts three hours. Available starting times depend on the schedule when you book.

Is hotel pickup included?

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

Can I swim inside the Keri Caves?

No. Swimming inside the cave is not allowed. The swim stop takes place in the surrounding Keri Caves area, usually near Oasi Bay.

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 food and drinks.

Will I definitely see a turtle?

No. The trip offers a chance to spot Caretta Caretta loggerhead turtles in their natural habitat, but sightings are not guaranteed. In July, some turtles may already have left because their eggs are hatching.

How much of Marathonisi beach can I visit?

Only a small part of the beach is accessible because Marathonisi is a protected turtle breeding ground.

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.