10. St. John, U.S. Virgin Islands

The smallest and least-developed of the U.S. Virgin Islands, St. John is 60 percent a national park — and 40 percent of that is underwater. Beginner snorkelers should opt for the 300-foot-long snorkeling trail in Trunk Bay, while in Salomon/Honeymoon Bay, you'll explore a slender reef teeming with vibrant brain, elkhorn, pillar, and lettuce leaf coral as well as fish, turtles, and rays.