Gladden Spit and the Silk Cayes are the big-water reason divers and snorkellers use Placencia as a base. The marine reserve protects part of the southern Belize reef, with clear water, small cayes, reef fish, turtles, rays, and seasonal whale-shark trips when conditions line up.
It is a boat excursion, not a casual swim from shore. Ask operators about season, sea state, and what is realistic; the reef is magnificent, but wildlife does not perform on command.