About the water taxi situation, there is usually a back-up boat if there are too many passengers. The association is very conscientious in accomodating as many passengers as possible, with the boat availability. You can check at the water taxi office when you arrive on the caye.
The best swimming and snorkeling from shore is at "The Split" http://www.cybercayecaulker.com/split.htm
Dont worry about crocodiles there.