The water taxis/ferries from Belize City take about 1 hour and 15 minutes. That includes a brief stop at Caye Caulker.
Probably the most dependable are the three daily Caye Caulker Water Taxi Association boats which cost US$12.50 one-way. Catch them at the Marine Terminal.
The Tropic or Maya Island planes are fine, but unless the water is rough the boats are fun, too. And about one-fourth the price of a plane from International.
I personally think it's interesting to take a boat over and fly back, or vice-versa.
Belize First Magazine http://www.turq.com/belizefirst/