We have been going to San Stef in May for the past 10 years. As it is the start of the season you are made most welcome wherever you go. Be prepared for slightly chilly evenings, there is also a possibility of a rainy day. If the Mistral, a cold wind from europe, starts blowing it can get rather chilly and it will keep you off the beach.
Just to add to that, it only usually blows strongly for a couple of days - 3 max, so it's a day by the pool if you don't want sand everywhere!!
but the Mistrals usually run in a 10-12 day cycle of windy weather with a rough sea & breakers on the beach, for a few days followed by a change to very calm weather with sea like a mill-pond, then a gradual slow build up to breezier weather again.
in my experience, so you uaually only see one in a normal fortnight, and often don't see any at all, we found it worse (if thats is a good word), in September than in May.
We have sat through quite a few Mistrals in the last 27 years & they have never spoiled our holiday, but worth knowing what to possibly expect!!
Whatever happens, it will not put you off this resort - we usually use it as an excuse to go off in a car for a day - or to visit Corfu Town!!