Coki Beach, on the northeast coast near the village of Smith Bay, is the best beach on St. Thomas for snorkelling directly from the shore. The water is clear, the reef begins close in, and the fish life is reliably good — parrotfish, sergeant majors, stoplight fish, and the occasional sea turtle moving through the turtle grass. It is a small, informal beach, and can get crowded, but the snorkelling repays the effort.
Adjacent to Coki is Coral World Ocean Park, an aquarium and marine park built over a reef with underwater observation areas and touch pools. It is oriented toward families and towards giving visitors without snorkelling experience a view of reef life they wouldn't otherwise get. Sea lion and shark encounters, a nature trail, and a semi-submersible reef tour are among the offerings. It's not essential if you're a confident snorkeller, but it's a well-run introduction to Caribbean marine life for those who want a guided experience.
The area around Coki is also known for informal food stalls and roving vendors, which makes it feel more local and relaxed than some of the bigger tourist beaches. Arrive early or late in the day to avoid the main rush.