Sunsail operate in Greece. Same 'family' as the Moorings. You willl find that cats are not as popular in Europe as U.S. and so not as much availability. September is coming to the end of the season. Water temperatures are, of course, warmer by then, but places could be starting to close down. Go right at the beginning of September, not the end of the month. Potentially more wind, rain and lower temperatures the later you leave it. European schools break July and August ( varies country to country, but mostly over these two months) so they are the busiest.
We have sailed many times in Mediterranean and Carribean. If you have keen snorkellers onboard, be aware that snorkelling does not compare anywhere in Med to the Caribbean. The fish just are not there in same quantity or quality. I am always disappointed. Had some good times in Greece despite that though.