Do what you feel is best, if you are trying to save money the water taxi only takes just over an hour and I can assure you you will not get wet. (unless it is raining and you are on one of the boats that is not covered.) Otherwise, I take the water taxi to/from the city on regular occasion and never get wet! If it is only one person you do not save much however if it is for multiple people you will. The $109 US Tropic (or Maya) is roundtrip to/from San Pedro.
From international you will pay $25 US to get to the water taxi terminal then $10 US to get to San Pedro. Here is the schedule for the Caye Caulker Water Taxi from Belize City to San Pedro.
4:30 PM Express