We arrived in June, which is after the big rainy season. In parts of Belize they receive over 200 inches of rainfall annually. The rain started coming down in buckets after our tubing excursion, but lasted only about 15 minutes.
Later, while riding back to the port, we passed through areas that hadn't received a drop of rain. You can actually see where it rained and didn't rain by the sharp contrast in how wet the ground was in one spot, while totally dry in another.
The weather in Belize was the hottest I've ever experienced. Just stepping outside caused me to begin dripping with sweat. Drink lots of fluid! There is purified, bottled water at every grocery and convenience store. Buy a couple of bottles whenever you can, just to have it around. You will need it. Do not drink the water from the tap.