"Trunk Bay is easy and inviting, even for first-time snorkelers," says guide Dave D'Alberto of OneHornButtfish (onehornbuttfish.com). "It's generally calm and clear, and there's a marked snorkel trail that follows a zigzag course in 3 to 18 feet of water." The park installed 15 underwater plaques along the trail that explain the ecosystem and identify marine life. Shallow rocky areas around the bay are home to octopuses and crustaceans, while the deeper water holds swarms of blue tang, yellowtail snapper, parrotfish and butterflyfish. For the more experienced "one-horn buttfish" (a local name for snorkelers, because all you see above water are snorkel tubes and rear ends), D'Alberto suggests circumnavigating Trunk Cay. "Start on the right side and swim counterclockwise," he says. "You'll see the young reef out there, with nice brain corals, and then ride the current back to the snorkel trail." In the middle of the bay, in 15 to 30 feet of water over sand and turtle grass, snorkelers often see green turtles, stingrays and eagle rays.