I agree, those are probably the only realistic choices for May.

this is a tentative itinerary with no set schedule yet , since the schedule will be available at a much later date.The time quoted for the trip duration from Piraeus was 5 hours.

http://www.igogreece.com/en/Greece.a...eas&id=127 Excellent information about Sifnos on this website. Eleni knows more about the island than anyone I know.