After reaching Greece, Persian king Xerxes I had moored his ships off the coast of Mount Pelion to wait out a storm prior to the Battle of Artemisium.
Hydna was well known in Greece as a skilled swimmer, having been trained by her father, a professional swim instructor named Scyllias, from a young age.
In another chronicle by Herodotus, Scyllias was actually working for the Persians as a diver, recovering a great part of the treasure sunk in the storm before deserting for the Greek side.
He supposedly swam submerged from Aphetae to Artemisium, possibly using a primitive snorkel or a stolen boat, and brought the Greeks information about the Persian fleet.
Scyllias appears in the 2014 film 300: Rise of an Empire portrayed by Callan Mulvey, though Hydna does not, being replaced by a male son named Calisto (Jack O'Connell).