I have access to a sailing catamaran from spring to fall, and when its not booked for charters we can take it out for a daysail or two... or more. There's still a cost to rent the boat, but at a substantial discount to the group.
The sailboat is located near Nafplio, so most of the sailing will be in the Argolic/Saronic -- much more comfortable there anyway when the meltemi are blowing.
This group is for all levels of sailors, locals and visitors to Greece as well. If you don't know how to sail but think its something you'd like to try, you're also welcome to join.