You actually have time to start the meds if you get it soon, ( you can get more info on the CDC website) but common wisdom indicates that you do not need it if you are staying on the caye and not going inland. As I understand it, the med does not prevent malaria, just starts treatment in case you get it, which is unlikely.
This may not be the same advice as CDC, but I have not heard of anyone contracting malaria on Ambergris Caye, and, if you did, I believe there are good options to treat it. In any case, if you are on your honeymoon, I advised minimizing stress and just avoiding worry. Use the DEET when you are outside. Oh, and congratulations, best wishes and enjoy your honeymoon in the perfect venue of Ambergris Caye.