We did a trip to Placencia Belize and were a little under-whelmed. The resorts were nice enough, and we weren't in five-star accommodations or anything, but take a walk down the beach a bit and things got (literally) trashy. My wife was pregnant at the time so we were a little more limited in our options, but if I were to do it again there are a number of options that allow you to rent places basically on private or semi private islands. If I were to do it again, I'd do something like
this, particularly if you're looking to fish.