You're right Kelley....
I haven't found one yet that is dedicated to Ambergris Caye weather. They all give Belize City weather which is totally different.
If it's any consolation...you may be in luck as rain tends to come and go quickly over the Caye due to the winds. Usually morning and evening showers that move thru pretty quickly.
It would be at the discretion of the dive operation or fisherman as to whether he deems it safe to go.
I'm sure it would just depend on the day...as he sees it....LOL
Have a fun trip!