That dark, playful body slithering through my legs in the warm waters of the Sea of Cortez is not a fellow snorkeler, but a California harbor seal.
The experience is both exhilarating - and a bit disconcerting. Just a few minutes earlier, before we jumped fins-first into the sparkling sea around Baja California's Espritu Santo Island, our guide had warned us about one particularly chummy elephant seal. He'd been known to wrap his powerful flippers around unsuspecting human swimmers and pull them down into the sea's depths.
The thrill of swimming among seals and witnessing close up the sea's rainbow bounty of fish, corals, and plants is just one factor in the tourism boom in Mexico's Sea of Cortez, the second-richest body of water in the world - biologically speaking - after the Red Sea.
More tourists are interested in experiencing the earth's rich biodiversity. But for areas like the Sea of Cortez, the boom is both a blessing and a challenge.
With the local population more dependent on economic activity that relies on preserving the environment, the impetus to protect and conserve is growing. But at the same time, the arrival of more visitors and more boats, plus the construction of roads and hotels, present the area with a growing threat.
"The second-biggest threat to species survival on these islands is the introduction of exotic species," says Mara Elena Martnez, a Mexican marine biologist for ISLA, a nongovernmental conservation group working here. "But threat No. 1 is uncontrolled tourism."
The trick, Ms. Martnez says, is finding the balance that allows tourism and nature to coexist: education of the local population, many of whom have some environmentally unfriendly ways; and education of tourists on their responsibility for the ecosystem's preservation.
On one rocky, cactus-studded island that is part of the Espritu Santo island nature reserve, a group of kayaking campers from Seattle attest to their efforts. But these repeat visitors also testify to the deterioration they've noticed.
"There's a surprising amount of trash around and fewer fish," says Phil Lynch, the group's leader. "But we're doing our part by taking out more trash than we bring in," he says.