Hot, humid and part of the hurricane season. We were in Belize 8/01 just after Chantel rolled through. Fortunately the storm itself was a non-event, however the massive evacuation of the island prior to the storm was not.
We had great weather while we were there - no storms at all. But, it was extremely sticky. August is also prone to what's referred to as "mauger winds" when the trade winds that typically help keep the place comfortable despite the heat, die away.
Odds are that you'll have a great time without problems, but it is a possibility.