I think you may have some difficulty in finding such a boat with no operator. The waters off Ambergris are not exactly the lake back home... lots of coral heads and the reef is very close. Boats and motors cost an arm and a leg to buy so the owners would rather not rent out their boats. But you should have no trouble finding a guide to take you out. They are all licenced and know the best spots to snorkel.
Have a fun time,