Found!Doulichion

Canali Stretti

Now there is in the dêmos of Ithaca a haven of the Old One of the Sea, Phorkys, which lies between two points that break the line of the sea and shut the harbor in. These shelters it from the storms of wind and sea that rage outside, so that, once within it, a ship may lie whithout being even moored.

Of course, it's all in the translation into English, different translators use different words and meanings, but this translation by Samuel Butler (revised version) is striking.
Here's what a modern scientist tells us about the marshy plains just behind the barrier:

The related marshy plain is separated from the recent beach system by the so-called Canali Stretti, a former channel leading from the central part of the Lagoon of Lefkada in north-eastern direction into the Bay of Aghios Nikolaos. It was used to navigate across the Lefkada Sound from Antiquity until the beginning of the 19th century.

This research by Simon Matthias May (and Andreas Vött) confirms the idea of an ancient breach right into the lagoon. The so-called Canali Stretti must have been used by Odysseus and Rhadamanthos. It might also be part of the route Cleopatra used to escape defeat in 31 BCE, but that's a story for another time.

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Canali Stretti

Odussey Book 13 line 93-100 Samuel Butler
e.g. Partsch, 1907