If your going to ambergris caye (San Pedro) consider the following. If not then disregard it all.
Use either aqua dives (try to go out with Roy/Russell) or Ambergris divers (If you can hook up with Ernest/Gonzalo best guide/capt tandem on the island). They are the cheapest and busiest shops in san pedro. Stay away from amigos del mar (way overpriced).
If you haven't done blue hole you should. We did blue hole as our first open water dive a few years back and it was spectacular.
Esmerelda is an okay site with a huge green morey (namesake of the site) but be careful, it is the closest site to the docks and the guides will take you there often if you don't speak up.
Hol chan is a neat dive with lots of fish/rays but really shallow. If your watching you budget it's almost as good as just a snorkel. The night dive here is worth it though.
Cypress tunnels and anything that ends in canyon is where you really want to go. If you can get someone to take you, the love tunnel is worth the longer drive.
The manitee snorkel/tour is a scam, don't do it. It consists of driving the boat to where the manitees are and watching from a distance, and then driving to another site to snorkel.
If you want to snorkel/dive with manitees, get Gonzalo at Ambergris Divers to set you up with Clifford/Justin. For the same money, they will take you to a site where you can dive/snorkel with the manitees.