one way boat fare to $AC is 14.00 so you would save $12.00 per person. Cab from Intl. to Belize City will cost $17.50 per cab, unless they have gone up in fares (you will need two). If you are in no hurry and if you will not have to rush to catch a boat, it may be ok, but if it is stormy or worse, raining, the discomfort may over-ride the savings.
When I do the boat route, I make sure I have some trash bags to cover my stuff that I don't want wet and I also have a towel and a fold up poncho.