Oahu, HI
Oahu's most popular snorkeling spot is this volcanic crater with a broken sea wall; its small, curved, 2,000-foot gold-sand beach is packed elbow-to-elbow with people year-round. The bay's shallow shoreline water and abundant marine life are the main attractions, but this good-looking beach is also popular for sunbathing and people-watching. Serious divers shoot "the slot" (a passage through the reef) to gain Witch's Brew, a turbulent cove, and then brave strong currents in 70-foot depths at the bay mouth to see coral gardens, turtles, and -- that's right -- sharks. (Divers: Beware the Molokai Express, a strong current.) Snorkelers hug the safe, shallow (10 ft.) inner bay that, depending on when you go, is like either swimming in a fish-feeding frenzy or bathing with 300,000 honeymooners. Because Hanauma Bay is a conservation district, you may look at but not touch or take any marine life here. Feeding the fish is also prohibited.