At mm28.5 there is a place called Dolphin Marina (http://www.dolphinmarina.net/index.html) that rents 20-22' CC. I have rented from there a few times and every time things worked out well. If you want to go snorkeling, you can go straight out from there and get to Looe Key. I believe its about 5-6 miles out. Easily found with a gps.
I was talking to a local there last time i was there and he was telling me about a local sandbar that they go to. It was close to the boat rental place but i don't exactly remember where he said.