From the Disney Website:
Gills and Thrills, at No Added Cost
Equipment for crossing Shark Reef, including mask, snorkel and life jacket, is included with your park admission, at no additional charge. The reef is 10 feet deep after the initial drop-off point, so participating Guests must be able to float across, either on their own or with a lifejacket. Children under 10 years of age must be accompanied by an adult.