I did a half day snorkelling tour with EZ Boy Tours of the Caye Caulker Marine Reserve.

Tours run from 10.30am till 1.30pm and 2pm to 5pm.
The tour spends one hour at three different locations:
Sting Ray Village
South Channel (Tropical Fish)
Coral Gardens

On my tour I saw sting rays, sharks, tropical fish, colourful coral, lobster, moray eels, barracuda and various fish like snapper.

E-Z Boy Tours can be found here