Diving the Sea of Cortez
Credit: Franco Banfi / Getty Images

Our guide warned us not to move, but there is only so much you can do when a fish six times your size comes barreling toward you underwater. I'd had two days to prepare for this moment – two days of horizon scanning and sonar reading – but reflexes are reflexes. I flailed. Fortunately, the 10-foot whale shark didn't mind. He was busing sucking up the plankton that thrives near the surface of the Sea of Cortez.

The sea, which Jacques Cousteau dubbed "The World's Aquarium," is a 68,340-square-mile peninsula of colorful coral and squid-infested deep water surrounded by the desert that runs north from Mazatlán and back south along Baja to La Paz, where divers like myself book rooms next to honeymooners and spring breakers. I'd been staying at CostaBaja, a luxury resort, and making day trips out into the glacier-blue sea.

My first trip was to Isla Espíritu Santo, the heart of a UNESCO-protected biosphere renowned for its reef life and friendly sea lions. I jumped in and watched playful pups play catch with sardine bait balls. When they grew bored with their game, they nibbled on my fins and arms and I let them.

My guides from Fun Baja got word that a whale shark was in the area while we were enjoying some grilled fish tacos and cerveza. Apparently, the fish was lurking just across the bay, but we never found it. We consoled ourselves with more cervezas, then went swimming with a pod of dolphins. That amounts to bad luck in the Sea of Cortez, but we all made peace with it while dolphin calves looped around us with their mothers.

The next day we settled for a manta ray. It winged past us and into the blue before we turned back for La Paz. We had made peace with the idea of a whale shark–free day when another outfitter passed along the good word. An adolescent was feeding nearby.

The fish was massive and its sun-dappled hide was spotted yellow and gray. It swam without effort but moved faster than any human ever could. Tiny feeder fish hugged its back and underside, swimming furiously to keep up. I emulated them briefly before giving up. The shark was what I had come to see and it was, as advertised, awe-inspiring. But, after the initial shock, it didn't seem unworldly or even frightening. In the context of the Sea of Cortez, that sort of scale and that sort of majesty seem normal – nothing to fear or flee from. I watched it disappear, surfaced, and had another cerveza.

More information: Go in late fall, winter, or early spring when temperatures become bearable. AeroMexico flies direct as of June 20, 2013, from Los Angeles to La Paz and vice versa twice a week. Rooms at the 550-acre CostaBaja Resort & Spa start at $200 a night.