Banco Chinchorro is the big reason serious divers look at Mahahual. The reserve lies offshore and protects a huge platform reef, with coral gardens, seagrass, mangroves, shipwrecks, turtles, rays, and crocodiles around the cays.
Trips depend on weather and permits, and the boat ride is long enough that this is not a casual swim from town. Go with a licensed operator, expect an early start, and treat it as a full-day reef excursion.