It's really a matter of preference and the number of days you will be diving. I just completed 10 dives in Maui, water temps were 78░, and because I was doing so many consecutive dives I chose a full 3-mil, just to ward off the cummulative effects that multi-day diving has on the core temps. Was perfectly comfy for all the dives. It also depends on how much "rubber" you carry with you on a daily basis (body fat), the leaner you are, the more chilled you can become and vice versa.
I've heard anything from diveskin to 3-mil full is acceptable for Belize. Enjoy!