What part of Belize will you folks be in John?
One of the most popular locations is Ambergris Caye, which is to the south. There are a lot of reasonably priced trips out of there. The deals there are some of the best in Central America. Check out
http://ambergriscaye.com/pages/town/fishing.html
Whatever you do, don't go to Belize City, or if you do, be very careful and only travel in the day in open and busy spots.
Drink only bottled water unless you want to spend most of the trip in the John. "John in the John" that's a good one, eh?
I don't know if you dive, but it is on the list as one of the top 10 diving destinations in the world. The blue hole is famous. Spectacular scenery and fish diversity.
I haven't been there in a while. From what I remember, some of the better fishing is in the South. As I said, Ambergris Caye is a good safe bet.
If you get a chance, try to visit Placencia, which is further S of that, and has some beautiful beaches where you can fish from the surf. As always, be safe and don't venture too far into any sketchy areas at night.
Good luck, hope this helps.
If you get any fish, post and show us. I would love to see some tropical scenery right about now. Stuck in an office and the rain is cold and dreary.