A flash of white, and the energy on board shifts in an instant.

A handful of Thailand Aggressor guests had taken to the top deck during a surface interval on dive day four of our North Andaman itinerary, lulled by the tropical heat, already thick at 9:30 a.m. We had spent the past three days diving the Similan Islands National Park, in the Indian Ocean off Thailand’s west side, and now were moored off the island of Koh Tachai.

Then we hear squeals. A passenger from Alaska pops from her lounge, pointing to the band of blue between us and the island. There on the surface is the unmistakable bicolor kite shape of an oceanic manta, which can range up to 23 feet across here, with 15 feet common. It lingers, breaking the surface with its wingtips. Everyone cheers.

Some might call it a wave; more likely, the manta is simply feeding at the surface—a good sign. This means it is probably here for the food, which won’t likely run out before we pop in for our next dive. We now know there’s an excellent chance we’re about to see something big.

See and Be Seen

I’m following black fins. The current is pushy, ready to carry us southward if we stop kicking. Jade Sooksombat, one of the onboard instructors and our guide for the dive, promises to take us to the north pinnacle, where the manta had been spotted. I stay on his heels as he winds around bend after bend.

Forty minutes and 1,800 psi later, we’ve mixed with giant trevally, yellow-back fusiliers, blue-dash fusiliers, longfin batfish—but no mantas. Just as Sooksombat points three fingers into his open palm to signal the safety stop, I see a patch of darkness overhead. Before I can reach my metal clip to bang my tank, I start moving, keeping pace with the manta as it glides before me.

I can’t help myself. The beauty of this animal is overwhelming—if this is our only encounter, I want it to last just a little longer. The rest of the group catches up for a few moments before it slips from view. Then we wait in the blue, hovering at 40 feet.

A few minutes later, a second manta, smaller, appears and seems to swim right toward me. It carries itself slowly, with almost imperceptible movements, the tiniest flutters. Then it draws near. To hold eye contact with an animal of this size is thrilling. It doesn’t look away, sizing me up as I size it up, a mutual mingling of curiosity and wonder.

Mantas, as it turns out, are much smarter than many divers realize. This large-brained animal can recognize individuals. Perhaps even more impressive, it’s also one of only a handful of species— along with elephants, dolphins and humans—that can recognize itself in a mirror. What this means is that meeting a manta feels like a greater connection than meeting, say, a whale shark—another pelagic spotted often on Thailand Aggressor itineraries—and that’s because it is.

To see either animal is exciting, but I’m more grateful to have time with mantas, an animal that truly sees divers. To be watched, studied by an animal so intently, is somehow so satisfying. In today’s reality of smartphones and constant stimuli, where most of our interactions with each other are full of distraction and disconnection, here, now, with this animal, feels every bit the antidote.

Sex and Murder

Of course, it’s not a new idea that animal encounters in the wild—or even simply being in nature—ameliorate the stress of daily life. Anyone who dives likely cites this as a reason to stay hooked on compressed air.

But what Thailand serves up is something altogether different. I’ve been diving for 26 years now, and, admittedly, I sometimes catch myself thinking about to-do lists when I’m looking at reefs. Sad, but sometimes it feels as though if you’ve seen one parrotfish, you’ve seen them all.

I did not catch myself doing this once in Thailand. Perhaps that’s because diving in Thailand is weird. Take Monday, dive day two of the trip. On the first dive, we drop in at Hin Pusa, aka Elephant Head Rock, off the tip of Koh Ba-Ngu. Like much of the topography on the first half of the trip, the dive weaves among granite boulders covered in coral. Pretty? Undoubtedly. Mind-blowing? Ask again after I tell you what happened next.

Dive guide Dan McLain leads us over a boulder to a patch of rubble where a blue ribbon eel noses from its den with almost half of its body exposed. The shock of color and the odd, saucerlike flaps of its nostrils are engaging. I’m just settling in to watch when I catch a flicker of movement 10 feet away.

There, a day octopus stops suddenly—halting all movement to avoid detection. But it’s too late; I’ve already spotted its telltale mottled skin, and it seems to have noticed. It unfurls its arms and begins oozing forward across the rubble again. This time, it swirls to reveal a torpedo of smooth white under its body. This hunter has taken down a juvenile whitetip shark and is lugging its kill back to its midden. Only, upon reaching the hole, the octopus realizes its dinner dwarfs its dwelling. The shark isn’t going to fit. The octopus spins a bit, allowing the whitetip—its gills flapping quickly—to displace more sand from the entry. With each frantic breath, the shark seals its fate.

But wait. It gets weirder still.

Three yards away, a second octopus blushes crimson then pales before slowly making its way toward the action, stopping just a few inches from the lair.

The animal is still save for one arm unspooling toward the other octopus. It’s as if this arm is independent of its host. The arm scales up the hunter, and slides into its ear hole—technically the mantle cavity, but it looks like an ear hole. Only this arm—the hectocotylus arm—is making contact.

I have no idea what I am witnessing. Maybe a tickle? Or perhaps one octopus is trying to gain purchase on the other to steal and devour the shark. Whatever is happening is mesmerizing, Octopus TV. I stay watching until I realize the rest of the dive group has moved on.

Back on the boat, more is revealed.

I tell McLain I assumed the two were competing for food. After all, the shark was a big score.

“You just witnessed something much rarer than that,” he says. “They were mating. The male sticks his tentacle into the female to make, er, a deposit.”

Cruise director Glenn Pearson chuckles. “Sex and murder at the same time!”

Crazy as this is to witness—a once-in-a-lifetime event—it’s far from the only intimate action we see during the week.

At Shark Fin Reef off Koh Payan, my dive buddy points at a cuttlefish, fixed in front of an overhang in the rock. The animal continuously unrolls an arm up into the pocket of the overhang, then returns the arm up under itself.

Again, I assume it’s eating, using its tentacles like utensils to scoop up yummy bits from that stretch of reef. Apparently I have food on the brain. Or maybe it’s because the chef on the boat keeps serving us mango sticky rice, addictive green curries, fried rice and other dishes that I can’t get enough of. Only my wetsuit is suggesting I not have seconds at every meal.

Back on the boat, the dive guides reveal what we have witnessed. Yup, more mating.

“She’s taking the eggs and stashing them up under the rock for safe keeping,” says McLain. Never have I seen this behavior, which made me wonder if I ever will again. It’s magic—and yet just one more once-in-a-lifetime experience on this sailing.

Rockin’ in the Wee World

On Thursday, the diving reaches a crescendo. We moor at legendary Richelieu Rock, hailed not only as the best dive in the country but one of the top 10 dives in the world. It answers to a slew of nicknames, including “Whole Roll of Film Rock,” referring to pre-digital days when the abundance must have driven photographers, limited to 36 frames, mad.

That abundance is why every Thursday, Thailand Aggressor spends a whole day—four dives—dropping in on the croissant-shaped pinnacle 124 miles from shore. It’s an oasis in the desert, the only underwater mountain for miles.

“We have no idea how marine life gets here,” says Pearson.

As in, how did reef fish know to establish themselves on this outpost? How do anemones reach this site? Somehow, a slew of marine life has ended up here, packing themselves in.

“If you can see it in the Indian Ocean, you can see it at Richelieu,” says McLain as he briefs us on how to maneuver around the site, which also has channels cutting through the interior.

Because it’s so far out in the ocean, it’s also heavily subjected to current. But, so is much of Thailand diving. Says Pearson: “You know the drill. Don’t expect to dive here without current.” While the crew shows us how we will dive the site to ideally stay out of the current, they’re also walking around with fish ID books, showing us what we might see: Clark’s anemonefish, tomato clownfish, Janss’ pipefish, ornate ghost pipefish, white-eye morays.

“We could go all day going through fish books, but at some point, I have to shut up and let you dive it,” says Pearson.

And so we drop in, heading straight to an edge of the reef where our guide had assured us we would find critters worth the swim. There, at 92 feet, a tigertail seahorse and two ornate ghost pipefish are all enjoying the same patch of reef. Neither moves much, and each is its own work of art to take in. Especially the pipefish, a true oddity, elongated with wide paddles for fins and a tail, all ending in pointy spikes—like Edward Scissorhands. All those pointy bits help the species disappear among the brightly colored crinoids it calls home. From there, we wind around to see a ledge on the reef, covered in anemones, each with a tomato clownfish keeping house.

As the dive unfolds, species after species lifts from the ID books and into our logbooks: free-swimming flatworms, a pair of orangutan crabs clinging to a sea fan down in a sand channel. The site is so dense with color, texture and movement that you could devote a tank to one small section and be happy, but of course we are here to take it all in.

Midway through the dive, I drop back off the reef to watch from a greater distance. Before me, a massive curtain of glassfish covers the site, seemingly operating as a single being. It’s hypnotic.

And really, this whole week has been—from the mantas to the cuttlefish to the octopus sex-murder mayhem. As I watch the swirl of life vibrate around Richelieu Rock, I know one thing for sure: I will be leaving this trip under Thailand’s spell.

Need to Know

When to Go
Thailand Aggressor operates November through April. Those looking for deals on flights should consider traveling in mid-January onward. If you’re tacking on time on the mainland and want to participate in a Thai holiday, consider traveling to experience Songkran in mid-April, which typically includes streetwide water fights. Or Loi Krathong, the Festival of Lights, when thousands of paper lanterns are powered by candles and released to the heavens on the night of the full moon in November.

Conditions
Water temperatures can vary between 72 and 82 degrees. This season, it’s been 83 with pockets of thermoclines averaging 76 degrees. Visibility ranges greatly. The average of late has been 45 feet; the best is 110 feet. Always be prepared to dive in current. However, no currents in Thailand are super fast or require use of reef hooks.

Recommended Training
Aggressor Adventures requires only an open-water certification for guests joining Thailand sailings. That said, due to the strong and somewhat unpredictable currents, an advanced open water certification or experience with current is recommended.

Operator
aggressor.com

Price
A seven-night sailing starts at $3,290.

Travel Tip
Don’t forget a safety sausage on a reel. If you finish a dive early or get separated from the group due to current or limited visibility, you will want to alert the tender drivers to your position.

Travelers who want to arrive in Phuket a few days early to acclimate to the time can venture to Khao Sok National Park, where pillars of limestone formations rise from Cheow Lan Lake. Find it 90 minutes by car north of the port. Bonus: This is a place where you can see elephants, tigers and gibbons in the wild, and can overnight in huts along the lakeside. Or, arrive in Phuket, then make the drive to Khao Lak, a town along a quiet stretch of golden sand coast. One highlight in the areais dinner at the restaurant Phu View, overlooking the town and ocean.

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• 15 Underwater Photos from Thailand | Open Wide and Say Ahhhh!