Beginner divers will appreciate the still water and vibrant sealife on Isla Mujeres ’s western side, while the east side presents more challenging options for advanced divers, with deeper water (up to 40 meters/131 feet), more varied terrain, and even a couple of shipwrecks.
Favorite sites include La Bandera (a reef dive), Media Luna (a drift dive), Ultrafreeze (a shipwreck, in notoriously chilly water), and the famous Sleeping Shark Cave—a deep cave known to attract sharks where they fall into a strangely lethargic and nonaggressive state. Explanations for this last phenomenon vary by the teller: Salinity of the water, low carbon dioxide, and underwater currents are some usual suspects.
Unfortunately, overfishing (and overdiving) has disrupted the slumber party, and there’s only a 50–50 chance, at best, of seeing sharks on any given day. September seems to be the best month, but you just never know.
Isla Mujeres’s dive shops charge fairly uniform rates: around US$40–50 for one tank, US$65 for two, and US$8.50–12 for equipment if you need it. The Sleeping Shark Cave and deep dives run a little higher. Open-water certification courses cost around US$350, including equipment and materials.
Aqua Adventures (Calle Hidalgo btwn Avs. López Mateos and Matamoros, tel. 998/877-1615, www.diveislamujeres.com , 9 a.m.–9:30 p.m. Mon.–Sat., 5–9:30 p.m. Sun.) goes the extra mile to provide friendly service and a safe, professional diving environment. Boats can carry up to 12 divers, though no more than 6 divers are assigned per dive master. The shop also offers the full range of courses, plus gear storage and night diving.
Sea Hawk Divers (Av. Carlos Lazo at Av. López Mateos, tel. 998/877-1233, www.isla  mujeres.net/seahawkdivers, 9 a.m.–10 p.m. daily) is a highly recommended dive shop owned and run by island local Ariel Barandica. Service is friendly and professional, and dive groups are never larger than six. Sea Hawk also offers accommodation in a number of rooms above and behind the shop (US$75 s/d with a/c, US$95 studio with a/c and kitchenette); they’re often booked but are worth asking about.
Formerly known as Coral Scuba Dive Center, Carey Dive Center (Av. Matamoros near Av. Rueda Medina, tel. 998/877-0763, www.careydivecenter.com , 8 a.m.–8 p.m. daily) is a recommended dive shop that enjoys lots of repeat customers. Equipment is in top condition, and prices are somewhat lower.