The cab fare to the Marine Terminal from the International airport is US$17.50 (that's for the cab, not per person,no tip necessary) and the one-way fare on the water taxi to San Pedro is US$12.50.
If the weather is calm and sunny, it's a nice boat ride, but if not, not.
If you are traveling with several people, you might also look at flying out of Muncipal, as the fare to San Pedro is about one-half that from International. Taxi fare from International is the same as to the Marine Terminal.
--Lan Sluder www.belizefirst.com