$100 us is a good rule of thumb if, like me, you can daily drink half your body weight in Belikins. ATMS are tricky in the country. All dive shops and tours [and most restuarants] will take Trav checks and credit cards. If your spot has a bar/restuarant then you can run a tab [cash for tips?] and payout at the end. If you've never been to a Mayan ruin, look into that tour as Lamanai, IMAHO, is epic.
Weather usually clouds over part of the day, not all day.