I stayed at Robert's Caye last summer for a couple of nights. There is some pretty good snorkeling within a few hundred feet of the island, easy swim distance, though I agree with Katie that of the two Ranguana has much better snorkeling.
The island is TINY, literally just 100 or 150 feet across, more or less a spit of sand. The cabanas are simple but pleasant, nicer than the cabins on Ranguana, and are built partly over the water, and you can walk in to the water from your deck, down some steps.
--Lan Sluder