This is from Mr B's site (http://belizebus.wordpress.com/)
Water Taxi
A water taxi is a boat, with seating for up to about 80 passengers, providing transportation across water. Water taxi trips are sometimes cancelled in rough conditions.
- Water taxi services between Belize City, Caye Caulker and Ambergris Caye (see websites for schedules & fares)
a) Caye Caulker Water Taxi Association:
http://www.cayecaulkerwatertaxi.com
b) San Pedro – Belize Express Water Taxi
http://www.belizewatertaxi.com/
c) San Pedro Water Jets Express:
http://www.sanpedrowatertaxi.com/index.html
Here’s another service that transports passengers from the international airport to Ambergris Caye:
http://www.tropicferry.com/
From what I understand, this is more of a door-to-door service. Tropic Ferry offers pick-up at the airport, private a/c shuttle transport to their boat (a 3 minute drive to a marina located across the highway from the airport), boat transport directly to your dock on Ambergris Caye, and they throw in all the rum punch you can drink.