Elbert, last time I took the water taxi there were 4 of us. Taxi ride from airport was $25US and I think water taxi was $15US. That is the best I can recall and I could be wrong, but it was definately cheaper.
Shuffles, I have taken the water taxi at least 6 times. NEVER have I had to lug my suitcases with me while walking around Belize City. They were well-tended by the water taxi terminal staff. Once when the terminal was kind of crowded I paid a young man to watch our bags. We travel pretty light and Camera, cash, etc are kept in my purse with me. Everybody's comfort level with this sort of thing is different.