By Bronwen Dickey True to the 16th-century explorer for whom it was named, northwestern Mexico’s Sea of Cortez (also known as the Gulf of California) inspires discovery and adventure. The narrow gulf is flanked on both sides by the painted Sonoran desert and dotted with dozens of small, uninhabited islands where sea lions and blue-footed boobies make their colonies. And while the cactus-strewn landscape above water might be rocky and arid, underneath the terrain is anything but desertlike: The warm waters separating Baja California from the Mexican mainland provide a nutrient-rich haven for more than 800 species of fish and many large migratory marine animals, such as whale sharks, flying mobulas, turtles, dolphins and several species of whale. If that doesn’t necessitate an expedition, what does? Now divers can explore the Sea of Cortez’s natural treasures via a new live-aboard, the 110-foot Rocio del Mar. The boat is owned and operated year-round out of Puerto Peñasco, a small Mexican fishing town, by Francisco “Lolo” Sandoval (a longtime boat captain) and his wife, Dora (an avid diver and dive instructor). The Rocio del Mar was custom built with the diver in mind: It features 10 air-conditioned staterooms with private baths, large open patios, a media room with leather sofas and convenient little extras like showers on the dive platform and plenty of wet-gear storage. But the real advantage of a trip on the Rocio del Mar isn’t the ship itself, but where it takes you. It is the only live-aboard in the Sea of Cortez that cruises through the Midriff Islands (so named because they sit near the middle of the gulf) on its seven-night itineraries, so you’ll have the opportunity to kayak along the unpopulated desert coastlines and hike into bird sanctuaries when you’re not fin-to-fin with the big animals for which the area is famous. Because most other vessels stay farther south, it’s possible that you won’t see another boat the entire week. The Midriff Islands have long been popular with fishermen because of their large numbers of yellowtail, grouper, dorado and big bass, but they are virtually uncharted territory for divers (which is, of course, the allure). Sharp tectonic shifts have created a craggy underwater geology full of steep drop-offs, submerged cliffs, soaring pinnacles and seamounts throughout the island chain where currents can get fairly strong depending on the season, but there are also numerous sheltered coves for novices. Most drift dives — especially off the larger islands like Isla San Lorenzo and Isla Àngel de la Guarda — begin around 30 feet and descend to nearly 100 feet, though the reef walls continue down much farther. And after your diving is done in the evening, you can kick back on the patio with an icy margarita and enjoy the dramatic, ever-reaching Sonoran sunset. Dawn to dusk, each day on the Rocio del Mar offers plenty of ways to rediscover the New World.

Need to Know

Travel Tip Charters run from Saturday to Saturday, with diving Sunday through Friday. Because the boat departs at 4 p.m., plan on checking in between 1 p.m. and 3 p.m.

Time to Go Year-round.

Dive Conditions Water temps climb to 85°F by the end of summer and dip into the low 70s during the winter. Currents and winds can sometimes pick up significantly, so it’s best to have some diving experience (though the staff is happy to cater to your level of comfort when choosing sites). A 3 to 5 mm fullsuit is recommended.

Getting There Plans are in the works to add Puerto Peñasco to several commercial airline routes, but for now it’s easiest to drive there from Phoenix, Arizona (3 ½ hours one way). Be sure to factor in extra time for border crossings.

Price Tag $499 for a three-day weekend trip, $1,995 for a seven-night charter (all rates are per person and subject to a 10 percent tax). Price includes four guided dives per day, meals, snacks, beverages (alcoholic beverages other than beer and wine are extra), tanks and weights. Don’t Miss Whale-watching. Some of the world’s most majestic whale species — including the fin, sperm, humpback and blue — travel through the gulf on their migration routes. Chances are you’ll see several of them, regardless of season.

Itinerary:

Day 1: Boarding and briefing in Puerto Peñasco.

Days 2 & 3: Explore Isla San Esteban and Isla San Lorenzo where you can plan on meeting iguanas and seals on land, and grouper and yellowtail in the water. Night dives are usually planned for spotting lobsters and octopuses.

Day 4 & 5: Isla Partida, Isla Raza and Isla Salsipuedes — more than 90 percent of the world’s Heermann’s gulls reside on Isla Raza, as do large numbers of blue-footed boobies. The big excitement underwater is the fact that several shark species hang out offshore.

Day 6: Head for Isla Angel de la Guarda. On the western side of this island there is an incredible wall dive with sponge- and coral-covered shelves. And you’re sure to be surrounded by sea lions for the entire experience. To the south, there is a fast drift dive along Bento’s Wall, and in October and November, whale sharks hang out around the island as well.

Day 7: Return to Puerto Peñasco and disembark after breakfast.

If the Boat Fits

The Sea of Cortez might be North America’s Grand Central for large marine animals, but fortunately it isn’t as crowded with divers ... yet. Other operators do run trips there seasonally though, so if you’re interested in more luxe accommodations or in diving the well-known sites farther south, check out the following itineraries.

Solmar V — Operated out of Cabo San Lucas, the company’s founders have been diving the Sea of Cortez since 1984, so they have insider knowledge about the area’s secret spots. For one week in October, the 112-foot luxury vessel — whose staterooms feature brass and mahogany details — circles the southern part of the gulf. It stops at the Cabo Pulmo National Marine Park and the famous Marisla (“El Bajo”) Seamount, in addition to several smaller islands. Prices start at $2,095 per person for an eight-day trip. www.solmarv.com

Nautilus Explorer — Departing from Los Cabos, the 116-foot Explorer adds a visit to Socorro Island — famous for giant manta encounters, dolphins and hammerheads — to its May Sea of Cortez itinerary for a comprehensive 10- or 12-day luxury live-aboard experience off Baja. Prices start at $4,050 per person for a 10-day trip. www.nautilusexplorer.com