Captain Morgan's is a great place, we've stayed there twice. There are two pools, the main one that was seen on Temptation Island, and a newer one over by the timeshare units. The newer one has a swimup bar.
The previous posters were correct in that the best swimming there is off the dock on the north side. Get some snorkel gear because there are these adorable tiny fishes that swim under the ladder and the resort has sunk some tires and stuff to make an artificial reef to attract fish. Another good idea is to take a kayak over to the reef and swim around there.
(Be sure to have Charles make you a Dirty Banana drink. And OUR favorite place to eat on the island is up the beach at the Laguna Grill...try the Belizean Bliss!) Tell Primo that Rich and Wendy from Iowa sent ya.