can anyone who has been in the last couple of years, remind me how frequent ferries are to the main islands in mid-May?

Swedish, Finnish, Estonian, Danish, Norwegian, German, Italian, Spanish, Greek, Tunisian islands [to be continued at will] ?

I'll assume that you mean from Piraeus out to the islands. In May to Santorini there are several different ferries leaving at different times and on different days during any given week. The same goes for the return to Piraeus. Look at these websites to see what I mean.

http://www.gtpnet.com/RoutesResults....GRSKY22,3125,6

http://www.gtpnet.com/RoutesResults....70PIRTZE,166,8

hsv - this was posted under the Greece thread so, so i thought that would be clear.
Anyway, Brother thanks for the links

There is no such thing as a Greece thread. Everything ends up on the Europe board. Only when you click on certain countries you might end up with that specific country's posts, but only if the poster indicated so.
Anyway, this will bring yourt message (now that it is clear) to the top again, so more chance for even more input. Cannot help myself, I am afraid.