Some of the myths are key to what we know about Greek history. For example, there’s a story told by Homer that indicates that the Greek Prince Paris, after seducing Helen of Troy and abducting her, fled the wrath of her husband Menelaus and they ended up hiding in this city, before making their way back to Troy.
The city also claims to have been host to the legendary Hercules while he was out doing his labours, and hence got the name Heracleion.
Both accounts, however, are taken by historians with a grain of salt. If there really was an island off the coast of Alexandria, there’d be evidence, right?