You can
anchor in the
lagoon at Spinalonga free of charge. I wouldn't leave the
boat there for more than a day unattended. The shelter is good, the local town (Elounda) is lively and there are good transport links to Ag Nik and Heraklion. The marina at Ag Nik tends to get booked up very quickly. We've spent the last four winters on board, at
anchor, in Spinalonga so if you want more info please ask :-)