Laurent, check out this site it will give you the weather patterns by month: http://www.belizenet.com/weather/climate.html Just remember, no matter the weather when you are on AC it is "just another beautiful day in paradise"!
Keep in mind that if your plans include snorkeling the winds may cause outings to get cancelled. I was there the first week of April, and it was still quite windy, but there was only one day when snorkeling trips got cancelled.
We do have plans for snorkeling and fishing, probably a catamaran tour too. Do you think those activities have a high risk to be canceled ?
I read that Feb-Apr are pretty much the same and are the 3 best months to go to AC. We could move our trip to April but we already booked most of it so we are not sure it makes sense to change everything now. Any take on that ?