The chances of coming down with something on a short visit to Cayo are slim, but better to be safe than sorry. There are several thousand cases of malaria reported each year in Belize, most in the far south and in border areas. Chloroquine is inexpensive, proven insurance.
It's always a good idea to have Tet and Hep A and Hep B, regardless of where you're traveling, although I understand that tetanus sticks are in short supply right now.
--Lan Sluder