The calm waters ringing the shallow reefs and tiny coves found throughout the island make it a snorkeler's heaven. The waters off the northeastern shores of French St. Martin have been classified as a regional underwater nature reserve, Réserve Sous-Marine Régionale, which protects the area around Flat Island (also known as Tintamarre), Ilet Pinel, Green Key, Proselyte, and Petite Clef. Equipment can be rented at almost any hotel, and most beaches have watersports kiosks.

One of St. Martin's best sources for snorkeling and other beach diversions is Carib Watersports (tel. 590/87-51-87), a clothing store, art gallery, and watersports kiosk on the beachfront of the Grand-Case Beach Club. Its French and U.S. staff provides information on island activities and rents kayaks for $20 an hour, paddleboats for $25 an hour, and snorkeling equipment for $10 a day. The main allure, however, are the guided snorkeling trips to St. Martin's teeming offshore reefs, including Creole Rock, an offshore clump of reef-ringed boulders rich in underwater fauna. The 1 1/2-hour trips depart daily at 10am, noon, and 2pm, and cost $25, with all equipment included. Reservations are recommended.