As you set out across the glassy shallows of Biscayne Bay toward the mangrove-covered humps dotting the horizon, you'll know why we continue to choose this spot for superlative snorkeling. Because the place is a national park, boat traffic and commercial activity are limited. The reefs have flourished. Tangerine-color clown fish, spectral triggerfish, and barracuda, are abundant. The snorkeling boat, run by a Hollywood company called Divers Unlimited, leaves once per day at 1:30 p.m. (For scuba divers another departs weekdays at 9:00 a.m. and weekends at 8:30 a.m.) The snorkeling trips return about 4:30 p.m. The cost of $27.95 includes rental of all equipment: mask, snorkel, safety vest, and fins.