One time, I was managing a Pacific dive site around a sunken World War II bomber. A couple complained that the guide was terrible and didn't pay attention to them. When I headed down and asked him how much air he had, he looked at me dreamily and began stroking his air gauge like a fluffy cat in his arms. He was totally blitzed.

When it happens to me, I either get the desire to dive deeper (because it's just so damned cool down there) or The Fear. I got The Fear once when a big-ass shark came swimming along below me in the Bahamas. Nothing kills your buzz quicker than a giant shark and nitrogen-induced paranoia. Not even when the security guard looks at you funny after you smoked up in the mall parking lot -- you're getting a cheese-filled pretzel; he knows.

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You probably picture coral as bright and colorful:

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And, yeah, I used to see plenty of coral like that. But on returning to the Bahamas in 2010, after previously diving there in 2006, I saw the coral had gone totally white, like this:

Duke

That's called coral bleaching, when coral lose their algae and starve. Temperature rise is one cause. Another surprising one: sunscreen. Sunscreen blocks the sun by design, but thousands of tons of it wash into the ocean every year. That keeps sunlight from penetrating the water, killing algae. Chemicals in the sunscreen also activate dormant viruses. I haven't worn sunscreen since 2008; I'd rather risk skin cancer than give it to the ocean.

Of course, it's not all terrible: One of the largest hermit crabs I ever came across was wearing a bright red shaving cream cap as his protective shell. It's a bit bittersweet to watch sea creatures using our trash to protect themselves. The cap likely didn't help him in camouflage, but that guy loved his plastic home. Floating flotsam is also used as safe havens for juvenile fish and sea life; often, they'll hide under a milk crate for protection, like they would with seaweed. It's strangely adorable, if you can look past how awful and sad it is.

Other fish and invertebrates can service you, too, though this isn't necessarily sexual (unless you start moaning all weird and make it that way). Cleaner shrimp and some species of fish have developed a symbiotic relationship with larger sea creatures, feeding on their dead skin and parasites. You can see cleaning stations along the reef in prominent areas, and some places are so popular there is a visible line up for service. Divers who are patient can have shrimp manicures, where the shrimp will pick bits off of your fingers and nails, and those who aren't too squeamish can go to the shrimp dentist.

Drew Kaplan

Diving is billed as a fun thing to do on vacation, but plenty of tourists never make it out of the water alive. It's way more complicated than it might look. A lot of math goes into calculating buoyancy, which is why divers don't float up against their will or plunge down endlessly. Poor buoyancy control either has you constantly on the bottom kicking up dirt, or else struggling to descend no matter how overweighted you are. No, I said you were overweighted. There's an important distinction.

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There's a lot that isn't fully understood about diving and the science behind it. Calculation tables for diving depths and times are still based upon U.S. Navy calculations, which use burly, muscular Navy men. That doesn't exactly apply to the 35 percent of divers who are women or the divers who are teenagers, middle-aged, unfit, and anybody else not shaped like a G.I. Joe.

Sharks might be notoriously fearsome killing machines, but they're also super shy. Like Michael Myers standing awkwardly in the corner at a junior high dance. In Thailand, I came around a bommie (an upward-growing outcrop of coral) and surprised a 2.5-meter leopard shark that was sleeping on the sand. When it realized I was there, its first reaction was to swim away. Large sharks can kill people, of course, but not because they want to eat us. They're just totally confused about what we are. You don't smell or swim or feel like a fish -- so what the hell are you doing in the ocean? I've spotted sharks in about 60 percent of my 2,500-plus dives and haven't encountered serious danger with any of them. The coolest sighting was in the Maldives, a school of 70-plus grey reef sharks feeding and circling. The smallest of them were 7 feet, while the bigger ones were anywhere from 10 to 13 feet.

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The moray eel is legitimately terrifying -- their dual jaw system was the inspiration for the mouth in the Alien series. But behavior-wise, they're like dogs and might even cuddle you if you come bearing gifts. Some eels love their human buddies so much that they recognize them over the years and develop a lasting relationship with divers who return to the same sites:

"Did you say outside? I wanna go outside!"

Aw, who wants to play fetch with their creepy second Alien mouth? You do, Murray The Moray! You do!

Lionfish look incredible -- they're just the sort of critter you'd reach out and stroke if you didn't know any better. But they're an invasive species and extremely venomous to boot. They were accidentally introduced to the Caribbean ecosystem and are now attacking the environment. And it's the divers who are fighting them.

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The crown-of-thorns starfish is another looker, but its venom hurts worse than a cigarette burn, and it sits in you for weeks. Its main predator is the conch snail, but humans like to sell their iconic shells, so the stars are multiplying like crazy. I've seen a swarm of them take down a whole reef in just weeks. I once hired a special team of hunters to bring them down. They'd go out five times a day and bring back 70 to 100 in a single dive. We had to bury them on land, because they're almost impossible to kill. Cut them apart and leave them in the water and they'll regenerate, their broken bits budding off into offspring.

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At least it has the decency to warn you about its crazy immorality by looking like a wicked video game boss ...