Scuba diving is excellent around St. Martin, with reef, wreck, night, cave, and drift diving; the depth of dives is 6m-21m (20 ft.-70 ft.). Off the northeastern coast on the French side, dive sites include Ilet Pinel, for shallow diving; Green Key, a barrier reef; and Tintamarre, for sheltered coves and geologic faults. To the north, Anse Marcel and neighboring Anguilla are good choices. Most hotels will arrange scuba excursions on request.

The island's premier dive operation is Marine Time, whose offices are immediately adjacent to the West Indies Mall, Chemin du Port, Marigot (tel. 590/87-20-28). Operated by Englishman Philip Baumann, it offers morning and afternoon dives in deep and shallow water, wreck dives, and reef dives, at a cost of $45 per dive. A resort course for first-time divers with reasonable swimming skills costs $75 and includes 60 to 90 minutes of instruction in a swimming pool and a one-tank dive above a coral reef. Full PADI certification costs $400, an experience that requires 5 days and includes classroom training, sessions with a scuba tank within the safety of a swimming pool, and three open-water dives. Snorkeling trips cost $25 for a half day or $80 for a full day, plus $10 for equipment rental. This organization will also arrange rentals, with a skipper and crew if necessary, of yachts and catamarans for jaunts to neighboring islands.

St. Maarten's crystal-clear bays and countless coves make for good scuba diving as well as snorkeling. Underwater visibility runs from 23m-38m (75 ft.-125 ft.). The biggest attraction for divers is the 1801 British man-of-war, HMS Proselyte, which came to a watery grave on a reef 2km (1 mile) off the coast. Most of the big resorts have facilities for scuba diving and can provide information about underwater tours, photography, and night diving.