By the time you realize you're being hunted, it may be too late to escape. The bull shark is about six feet long, weighs nearly 200 pounds and is capable of speeds up to 20 miles per hour.

Imagine meeting one in the water. You've done all you can to protect yourself, jamming your back hard up against the wreck you were just diving. You're gripping your spare air tank like a baseball bat and darting your eyes for any sign of incoming jaws. You're utterly still, save for the pounding in your chest, praying that the bubbles escaping with every breath won't draw more attention.

"The worst thing you can do if you see a shark is act frightened and splash around," says Dr. Michael Jacobs, an internal medicine physician on Martha's Vineyard, co-author of "A Comprehensive Guide to Marine Medicine," and a lifelong boater. "That means underwater or in shallow areas, where some sharks are known to hunt. If you can protect your back, then you have a chance because they like to attack from the side or back. If you can see one coming, you can hit it on the snout, although I'd say "˜good luck with that' if it's a great white."

Stings from animals like jellyfish are more likely to be a menace than sharks.This scenario is certainly terrifying, but in reality sharks are an unlikely encounter. Attacks are relatively rare, occurring only about 100 times a year worldwide. Great whites, bull sharks, and tiger sharks are believed to be the most aggressive, with bulls being found from Florida to New Jersey, tigers swimming around Hawaii, and great whites preying from Australia all the way to North America's west coast.

Fewer than 10 shark attacks each year are fatal – and many of those are due to hemorrhaging caused by the bite. "If you can stop the bleeding, in a lot of cases, you'll have a chance," says Jacobs, who spoke at the week-long "Medicine for Mariners and Safety at Sea" conference at The Bitter End Yacht Club on Virgin Gorda in January.

MARINE CREATURES THAT STING OR PUNCTURE

Your odds are actually far greater of a nasty run-in with a stinging animal, which can be just as deadly as any shark. Steven Spielberg hasn't yet made a horror movie about stingrays or jellyfish, but in severe cases, their venom can cause anaphylactic shock, respiratory paralysis or stop your heart.

"Boaters tend to think of jellyfish as causing prickly and tingly injuries, but there are some, by the sheer potency or amount of venom they release, that can kill you," Jacobs says. "It's like you're getting an injection of venom."

About 100 of the 9,000 or so known species of stinging marine animals are dangerous to humans, he says. The challenge in treating sting injuries onboard is that the treatment is different, depending on the type of animal, and there is a risk that you can make the sting worse. Jellyfish, for instance, hurt you because their tentacles contain millions of venomous stinging cells called nematocysts. Each nematocyst contains a tube that shoots out and releases venom, like a needle injection directly into your skin. The venom can go straight to your bloodstream from nematocysts that have released their venom tubes, while other nematocyst cells remain coiled and ready on your skin.

Stings from the Portuguese Man O War must be treated specially.Most jelly fish stings need to be flooded with salt water, and then require immediate dousing in vinegar (5% acetic acid), or another chemical to deactivate the venom.

"Fresh water will activate the disharge of more nematocysts," Jacobs says. "That's why for jellyfish stings, you want to flood the sting with vinegar. Never, ever rinse with fresh water, and never rub the wound or use your bare hands to touch it. .Portugese man-of-war stings are the one exception to this treatment. They are uniquely treated with hot fresh water (not salt water and vinegar) for rinsing off the nematocysts.

Also forget the myth of urine easing a jellyfish sting, it is not likely to be effective.

"You don't want to add to the person's trauma," Jacobs says. "For instance, a lot of people might think to put a cold pack on a sting. Think about that. You're on a boat, in the sun, applying a cold pack. There's going to be condensation. That's fresh water. If it's a jellyfish sting full of nematocysts, you're taking a bad injury and making it worse."

Stingrays, on the other hand, don't get you with a tentacle full of stinging cells. Instead, they have a spine on their tail that swings around and punctures your skin like a bayonet, sometimes breaking off and continuing to release venom after the actual encounter with the animal is over. Steve Irwin, the wildlife expert and host of "The Crocodile Hunter" television show, was killed by a stingray barb in 2006, though that was a severe case in which Irwin's heart was pierced.

True to its name, the stingray has a barbed tail that stings.As with most puncture wounds from marine animals with no nematocysts to worry about, you want to rinse the wound in fresh water – and then immerse the injured area in hot (110º-113ºF) fresh water for 30 to 90 minutes to deactivate some of the proteins in the stingray venom.

STING SYMPTOMS

In all stinging situations, you need to be aware that the victim can go into shock, Jacobs says. The symptoms you might see after a sting can include intense pain, nausea, vomiting, bleeding, weakness, headache and shortness of breath. More severe symptoms are seizures, cardiovascular collapse and paralysis. If the symptoms appear to be life-threatening, call for help immediately. Assuming the person is conscious, breathing, and relatively relaxed, the first thing you want to do after treating the sting is remove any remaining nematocysts or spine fragments that might continue to release venom.

In the case of a jellyfish sting, use tweezers first, then gently shave the area (use a knife, shell or credit card if you must) before applying more vinegar. In the case of a stingray puncture wound, after soaking in hot water for at least 30 minutes, remove any fragments that you can and wash the area with soap and water to prevent infection. A trip to an emergency room will be necessary to throughtly clean the wound.

"When you think about it,, bacteria really are the most hazardous of all marine creatures," Jacobs says. "A bad infection can kill you just the same as a shark if you don't do the right thing."