Excuse me but the island of Ambergris Caye has some great beaches to see and a few even excellent for swimming. The most impressive beach I've seen on the island is at Tranquility Bay in the Basil Jones area, wow, awesome snorkeling and a powder white sand beach sloping from twenty feet high. A short hike north will bring you to Robles Point, where land and the Mesoamerican Barrier Reef System meet, the only place on Earth this happens.
Mar de Tumbo (Banyan Bay) has a killer beach, crystal clear water, shallow, hard sandy bottom and the best part is a huge sand bar about 100 yards off shore with depths from six feet to six inches.
Ramon's has a pretty nice beach too.
Placencia's beaches are nice too, but different than AC, to each his own. Whenever I swam in PL I got bit by sea roaches.