I dunk my face underwater and see a manta ray the size of a small car gliding toward me, its toothless maw vacuuming up a first course of plankton. My guide shouts, “Say hello to Lefty!” Lefty, who measures more than three metres across, somersaults almost belly to belly with me as I try to remain flat on the water’s surface and not break rule number one: do not touch the manta rays.

I had arrived at dusk at Garden Eel Cove, situated along the western shore of Hawaii’s Big Island. Manta rays, kite-shaped ocean giants, gather here each night to raid the plankton-rich pantry. Depending on whether they are reef or ocean manta, the fish have wingspans that can run three to seven metres across, and though they are related to stingrays, mantas have no stinger. Before tourism sparked interest in the early 1990s, little was known about Hawaii’s manta rays. Now a multimillion-dollar industry has taken thousands of divers and snorkellers to view the creatures.

Before getting in the water, I sit on a small boat with 20 other soon-to-be snorkellers and flip through a binder with photos and names of 188 local manta rays, including Lefty, who was the first manta ray identified in the area. The clientele ranges from a seven-year-old who tries to convince his mom to come in the water, pleading: “Mom, this is a once-in-a-lifetime opportunity!” to the boy’s grandparents, who happily squeeze on fins and masks.

Through the boat’s lights, I see white surf spatter the craggy shore 100 metres away. Just as I reconsider dunking myself in the pitch-black ocean, our guide throws down two circular flotation rings, each big enough to accommodate 10 to 12 snorkellers. We’re instructed to hold on Superman-style: arms outstretched, two hands clutching the outside of the ring, body floating on top of the water.

I jump in, an underwater flashlight dangling from my wrist, and take my place. With our guide working like Hercules to tug the Superman ring, we swim about 50 metres from our boat to where the mantas are known to congregate.

The lights of the divers on the bottom reach up to meet our flashlights. Everything seems to glow. Within minutes, five manta rays appear beneath us, twisting through schools of silvery fish. Lefty arcs up toward me, almost ghostlike in the foggy light beams, her white belly marked with a distinctive black speckled pattern akin to a human fingerprint.

For one second, I look straight into Lefty’s body through her open mouth. Inside, I see black and white markings that look like a skeletal X-ray. I learn later that this structure is the manta rays’ gill rakers, used for filtering the plankton from the water. Lefty crests an arm’s length beneath me, then dives deep. I realize I was holding my breath—not in fear, but in awe.

A pair of large blue swimfins nudges my face. I raise my head from the water and find myself surrounded by snorkellers attached to other rings. When I’d signed up for this excursion, I didn’t realize how large the industry was or how many boats would be out at the same time (I saw seven other boats). The price was a reasonable $89 for 45 minutes in the water. But now, looking at the web of fins, legs, and arms in the water, I wonder what it might cost the manta rays to have so many observers daily.

Yet I had learned more about manta rays in the past hour than I had in my whole life. Before we went in the water, our guides explained a set of guidelines that they, and many other tour operators, had committed to follow when taking tourists out to visit the rays. According to the Manta Pacific Research Foundation website, these were drafted by the Ocean Recreation Council of Hawaii and PADI’s Project AWARE, both of which promote responsible interaction with marine life. The guidelines were drilled into us; first and foremost: don’t touch the mantas. When the manta tours first became popular, “petting” by curious tourists was destroying their protective layer of mucous, resulting in pink patches. Those patches disappeared once the tour operators implemented the no-touching guidelines.

We drift away from the other tourists. More than a dozen manta rays now spin beneath us. Although mantas with 4.5-metre wingspans can weigh up to 1,400 kilograms, they move with the ease of acrobats. Our guide, who has spent countless nights watching these creatures, shouts out names as mantas barrel up toward our lights. As Big Bertha—who, at 4.8 metres across, is the largest identified manta in the area—rolls directly under our ring, the seven-year-old screams with delight. I feel the same way.

Our guide literally drags us away when our time is up. Though shivering and exhausted, I’m sad it’s over. It was easy for me to sign up to see the manta rays. It’s harder to know if I was being a responsible tourist.

Through some research after my return, I learned that for these Hawaiian manta rays, the celebrity attention helped in the creation of a 2009 state law that protects manta rays against commercial hunting, a boon for creatures that do not reproduce quickly and are highly valued for their “medicinal” gill rakers in Chinese markets.

Even so, the next time I consider tourism that involves animals, I will research the industry in advance. And if I’m lucky enough to see the manta rays again, I hope to get another close-up of Lefty.

Access: For more information about manta rays and responsible marine tourism, see the Manta Pacific Research Foundation, Manta Trust, a U.K. charity that promotes manta conservation, and the U.S. National Marine Fisheries Service’s marine mammal and sea turtle viewing code of conduct for Hawaii.