The islet of Marathonisi is placed across the beach of Keri, in Laganas bay. It is not a single island but also includes two more small islets called Pontikonisia. Its perimeter is just about a mile and can be reached by boat from Keri beach and Laganas bay.

In any case, visitors must be very careful during their trip to Marathonisi because this is a breeding place for the sea turtles Caretta Caretta, an endangered species of sea turtle that go to Marathonisi to reproduce in the summer season. This is why visitors are absolutely not allowed to explore the island, but they are expected to stay only in the first 5 meters of a small beach so that they do not harm the reproduction of the sea turtles. Moreover, another species in danger of extinction, the Monachus Monachus sea seal goes there to rest and reproduce.

Marathonisi is not inhabited and keeps its characteristic pine, olive and green oak forests in excellent conditions. It has thick vegetation everywhere and two main beaches with different features. One of them has white sand, while the other has pebbles. The sandy beach is where the sea turtle goes to lay its eggs and that is why Marathonisi is included as part of Zakynthos Marine Park.