TMM (Tortola Marine Management) operates out of SP with a few catamarans. Walk south from the Sunbreeze and the first real dock you come to (with a bunch of cats on it) will be theirs.
It's pretty shallow inside the reef (8-10 feet) so unless you really like to sit halfway up the mast looking for coral heads it's probably not a great idea to try sailing a keelboat in there.
On the other hand, you can rent a hobie at Ramons an have a great time going back and forth along the beach - beam reach the whole way (except to get out and in) it's an onshore breeze (ok, moderate gale) most of the time.
cheers
sandy