Lainie,
We took the water taxi from Belize City the Sunday after Kieth hit. The boat was packed with people and tons, and I mean TONS of supplies, luggage, water, just tons of stuff. It was piled up in the middle of the boat close to our shoulders and we would all kind of work as a team to keep it all in the middle of the boat and from falling on anyone.
The taxi also has a area in the front of the boat for some storage. I would suggest though if you do take the taxi you make your luggage water resistant. We had a squall come over as we arrived at CC, it only lasted about 15 minutes but everything got soaked. We (our clothing) dried out very quickly but we looked like drowned rats! The water taxi was great but if I were you; take some drugs and do the plane.
Hope