Atlantis tsunami!

…No, almost certainly not: 300 BC is well within the historical era. Still, this is interesting:

Ancient tsunami ‘hit New York’

A huge wave crashed into the New York City region 2,300 years ago, dumping sediment and shells across Long Island and New Jersey and casting wood debris far up the Hudson River.

The scenario, proposed by scientists, is undergoing further examination to verify radiocarbon dates and to rule out other causes of the upheaval.

Sedimentary deposits from more than 20 cores in New York and New Jersey indicate that some sort of violent force swept the Northeast coastal region in 300BC.

…if only because I don’t think that there are contemporary reports of anything happening on the other side of the Atlantic, although at that latitude… no, the Iberian peninsula had cities at that point; somebody would have noticed, say, a big honking asteroid hitting the ocean.

Ach, well: link it up with the legend of Ys and hope that your players only half-remember their Poul Anderson. Or that they like big honking asteroids more than they do accurate mythological references.