The Caye Caulker Water Taxi Association Boats leave Belize City for San Pedro at 9, noon and 3 and returning I think at 8, 11:30 and 2:30. Probably Marty (the owner of this site) has some ads for the other boats that make the trip.
Another option is to fly from the Municipal Airport. Flights about every hour or so, roughly Us$50 round-trip. You could boat over and fly back, or vice-versa.
Belize books and publications by Lan Sluder:
Belize First Magazine http://www.belizefirst.com/
Fodor's Belize and Guatemala Guide
Belize First Guide to Mainland Belize
Adapter Kit: Belize
Fodor's UpClose Central America
Belize Book of Lists 2000
San Pedro Cool (coming soon)