Clear blue-green waters, coral reefs, and swarms of multicolored fish make for good local snorkeling and diving, especially in little Puerto Angelito and Carrizalillo bays just west of town. A number of adoquín stores sell serviceable resort-grade snorkeling equipment.
A professional dive shop, Aventura Submarina (Av. Pérez Gasga 601 A, local cell tel. 044-954/582-2353, asubmarina [at] yahoo [dot] com [dot] mx, 9 a.m.–2 p.m.), at the west-end stop sign, has gained a foothold in Puerto Escondido [1]. It’s run by friendly veteran PADI instructor Jorge Pérez Bravo. Introductory lesson, including one offshore dive, runs about $80, including equipment. For certified (bring your certificate) open-water divers, a one-tank night dive costs about $60, a two-tank day dive, about $120.
Alternately, check out equally professional Puerto Dive Center (a block farther east, at the corner of Libertad steps, across from the HSBC ATM, local cell tel. 044-954/102-7767, info [at] puertodivecenter [dot] com, www.puertodivecenter.com [2]).
Links:
[1] http://www.moon.com/destinations/oaxaca/pacific-resorts/puerto-escondido
[2] http://www.puertodivecenter.com