IT WAS THE TURTLES that changed me. Green sea turtles, four feet long, 400 pounds, gliding through the saline solution 25 feet below the surface like chubby, hard-shelled aliens.

I'd come to Hawaii to become a PADI-certified Open Water Diver, with two days of pool-and-ocean-based instruction. The week before, in my living room, I'd completed PADI's new online eLearning course, involving about 12 hours of slides on water pressure, buoyancy, nitrogen, etc… . finally passing the last test one night at midnight. (Even bleary-eyed, it was hard to fail.)

Next I flew to Honolulu, where I'd arranged to finish the course with Ocean Concepts dive center. Although most beginners do the training in three days, I'd picked the two-day version, which meant completing all the pool skills plus two beach dives on the first day. Once I'd been fitted into an Aqua Lung wetsuit, mask, and fins, my instructor, Joanna, put me through three hours of mastering skills. The pool turned out to be as scary as anything that came after: Even just eight feet down, even with some snorkeling experience, doing the mask-removal skill was freaky: Close eyes, take off mask, hold it out, resist instinct to breathe through nose, replace mask, purge mask by blowing out nose, open eyes. Fortunately, Joanna was a patient coach. After a long morning, the idea of swimming through crashing surf to make two beach dives was daunting. But I finned out, took some deep breaths, and dropped down 25 feet, and as soon as I saw all the trumpetfish, butterflyfish, and a spotted eagle ray patrolling the neighborhood, any jitters dissolved in the current. OK, I thought, this could get very cool.

On the second day, we did two boat dives, the first one 60 feet down to a wreck we could make out clearly from the surface.

Despite earlier difficulty, my ears had no trouble equalizing. Again Joanna signaled me through a checklist of skills, practicing things like a fin pivot—inflating the buoyancy-control device just enough to lift slightly with each exhale—and I had time left to explore, brushing a bit close to a rusty hole through which a gaping-mouthed moray stared back like a satanic cartoon fish. Next we tooled up Oahu's west coast to a shallow reef, where I navigated with a compass. When I popped out on the surface I was certified, which meant I was free to dive anywhere in the world, and to advance to higher levels.

But it was the next day, swimming for fun through the Makaha Caverns, that I became a diver. As we dipped in and out of a series of overhangs and grottoes, there were dozens of species of polychrome fish and, just chilling on a ledge inside one cavern, a sea turtle. I saw another and followed it along a valley in the seafloor. And then, looking up through an archway, I saw a third. It floated down past me, then gently doubled back and looked me square in the mask. If I had anthropomorphic tendencies, I'd have thought he was delivering some "Welcome to our underwater kingdom" message. But being more of a realist, I expect he was probing my potential as a food source. Either way, it's a different reality down there.