Spanky: With very few exceptions one needs to go snorkling with a guide. The trips are inexpensive and in most cases include the use of mask, snorkle and fins. Prices range, based upon length of trip, number of snorkle stops and such from about $15 to $40 per person per trip.
The places that you can do quality snorkling
directly from the beach require that you get transported by boat to the particular Caye that has a close in reef and to my knowledge most of them are located in the Southern part of the country such as "Laughingbird Caye National Park" and the Silk Cayes which are
located off of Placencia. You would probably spend the amounts listed above just to get to them.