Athens: Its Rise and Fall Book III Vol II by Edward Bulwer-lytton

I. After the victory of Thermopylae, Demaratus advised the Persian monarch to despatch a detachment of three hundred vessels to the Laconian coast, and seize the Island of Cythera, of which a Spartan once (foreseeing how easily hereafter that post might be made to command and overawe the Laconian capital) had said, "It were better for Sparta if it were sunk into the sea. " The profound experience of Demaratus in the selfish and exclusive policy of his countrymen made him argue that, if this were done, the fears of Sparta for herself would prevent her joining the forces of the rest of Greece, and leave the latter a more easy prey to the invader.

The advice, fortunately for the Greeks, was overruled by Achaemenes.