December is a transition month - weather on any given day could be just about anything (except probably not 95 F - that level of heat does not generally happen in Dec).
Water temps should be good, nights will be cooler for sleeping.
We could have a cold front, a sunny day or a rainy day - or all three events in 24 hours.
The only December bummer really is that it's sand fly season on Ambergris Caye. Sand flies (or fleas - I never quite learned which it is)are those pesky bity tiny no-seeums that hurt when they bite and make you look like you have measles. THOSE guys.
They are mostly an evening thing - and can come right through the window screens. Even if it's not very hot your best defence is to close windows and turn on the AC. Outdoors, use deet if the "other" stuff does not work in repelling them.
Another excellent item - the "Vape" machine!! Buy in supermarkets here - they work. A small plastic box with a little metal plate on it - plug it in, place a "vape mat" (small piece of treated cardboard) on the metal, and it slowly warms and releases a pleasant smelling something into the air - voila, NO bugs. Worth every penny.
If you do get bit (and you probably will eventually) there are some excellent itch remedies - one is called "Itch-A-Way" and is straight hemp oil. The other is Jungle Salve from Rainforest Remedies. Either one of these items provides virtually instan relief. Both are available in San Pedro and probably at the Internatinal Airport in Ladyville (check the gift shops in the departure lounge). Buy this remedy before you need it so you can quickly apply it when the itch hits. You'll be glad you did.