Certified divers have some difficult choices to make when planning a trip to the Sea of Cortez. Wherever you go, you’ll encounter one of the world’s richest marine ecosystems, with sea lions, sea turtles, whale sharks, manta rays, amberjacks, and schooling hammerheads all living among the islands, seamounts, and bays in the sea. Water temperatures are comfortable most of the year; visibility rarely falls below 6 meters and can exceed 30 meters on the best of days.
For those divers who are seriously dedicated to the sport, liveaboard trips are another option to consider while visiting Baja.
Diving at Cabo San Lucas : If you choose to stay close to the lively scene in Cabo San Lucas, short boat rides are an advantage to diving here; crowds and noise from harbor traffic are the main deterrents.
A deep submarine canyon just 45 meters offshore offers exciting underwater topography for experienced divers. The canyon is known for its “sand falls,” streams of falling sand channeled between rocks along the canyon walls. But even when the rivers aren’t running, the dive offers varied marine life, large and small.
Another popular dive from Cabo San Lucas begins in the surge under a sea lion colony on the Sea of Cortez side of Land’s End and concludes with an underwater swim around to the Pacific side.
Diving at Cabo Pulmo : Alternatively, you can sacrifice nightlife and amenities to explore the living coral reef and national marine park offshore from Cabo Pulmo in the East Cape region. The coral attracts a wide variety of fish of all sizes and colors; other smaller reefs, as well as shipwrecks, lie in the general vicinity. Drift dives are common here, and boat rides are short. Several sites, including a sea lion colony, are ideal for novices.
Diving at La Paz : The islands and seamounts offshore from La Paz harbor some of the most exciting underwater topography and the largest marine creatures in the sea. Boat rides are long — an hour or more — and currents tend to be strong at these sites. The reward is the chance to experience the deep blue and to spot some of the largest pelagics in the sea. An advanced dive site called El Bajo is famous for its summer population of giant manta rays and, in summer, schooling hammerhead sharks. Beginners will enjoy dives in the bays around Isla Espíritu Santo  and the sea lion colony at Los Islotes .
From La Paz south, onshore water temperatures are around 21°C (70°F) in winter and 29°C (85°F) in summer and early fall. Water visibility is best July–October, when it exceeds 30 meters; this is also when the air temperature is warmest, often reaching well over 32°C (90°F).