I was there 4 years ago and the first thought I had of Belize was "Poverty". The cab ride was scary, an old 70's model Ford van with no a/c and every rattle known to man. The streets and town look like something right out of the pages of National Geographic. Small dirt roads, bars on the windows and kids darting across the street.
I think we stayed at a Sheraton, but it may have been the Marriott. The previous posts are correct, the hotels are right on the water, within feet actually, but it is nothing but large rock boulders. We decided to walk around the neighborhood, but quickly found out that it wasn't an area for pastey Americans.
The people are nice and love the American dollar. When I was there, the exchange rate was 2:1. The island resorts are Americanized and this is where you will want to spend your time, if you can. Lots of water activities, snorkling, scuba, etc. The island areas are just like America and they will cater to you.
Overall, not a bad experience, just be aware of your surroundings.