The boat captain dove into the water and began swimming furiously toward a man and woman treading water.

What’s he doing? We’re not in any trouble. Perplexed, the couple tried to wave him off.

But he raced past them and grabbed their nine-year-old daughter, who was drowning just several feet away. The parents had no idea she was in trouble. The captain lifted her so that her head was clear of the water. “Daddy!” cried the girl before bursting into tears.

It’s a story Mario Vittone heard from an acquaintance, and he tells it often. In fact, his story has gone viral in social media and reappears each summer during swimming season. For Vittone, it had a special resonance. “I had worked as a pool lifeguard as a younger man and I had done that myself — jumped past someone who is next to someone drowning but doesn’t know it.”

These days Vittone, a water safety expert and former U.S. Coast Guard off-shore rescuer, warns people that drowning doesn’t look like the stereotypical splashing and yelling. People don’t drown the way they do on TV and in movies.

Francesco Pia, a former life guard and water safety expert, first coined the term “silent drowning” after studying films of swimmers in distress to see how they really reacted.

Swimmers often pass through a phase of distress when they know they are exhausted and are headed for trouble. At this stage they may still be able to call out for help or grab a rescue device. But Pia found that the instinctive drowning response makes it unlikely that the victim will scream and flail around.

Swimmer or nonswimmer alike, once someone begins to drown, he or she will be able to struggle at the surface for only 20 to 60 seconds before sinking for good. In this stage someone may appear to be treading water, but as Vittone has said, “It doesn’t look like ‘Help, I’m drowning!’”

In some cases, silent drowning strikes almost as soon as someone hits the water.

“I’ve seen people slip into the instinctive drowning response simply by stepping into deep water. They don’t have the skill and immediately their airway is compromised,” says Vittone.

One software engineer from India who never learned to swim, in fact, recalls sinking to the bottom of the deep end at a company pool party. To his horror, he realized he couldn’t get back to the surface. Fortunately, a coworker realized that he had been at the bottom of the pool a suspiciously long time and hauled him out before he drowned.

Related: Lifesaving Water Safety Tips: Make Your Pool a Drowning-Free Zone

Signs of “silent drowning” that people tend to miss

Here are some little-known signs of drowning to watch out for on the beach, lake or poolside:

Gasping. Someone drowning can rarely call out for help, says Vittone. Physiologically, voluntary action is gone; the mind is focused entirely on clearing the airway. Speech — even shouting that could save a life — is no longer the brain’s priority, Vittone says. “If you’re not breathing well, you’re not going to waste any of that breath to make sound. So the only thing you’re going to hear from them is the gasping, which is pretty quiet.”

Bobbing. There’s another reason a drowning person can rarely call out: His nose and mouth aren’t above the water long enough to expel air and water, inhale and shout for help. Once someone is drowning, he alternately rises above the surface and sinks below it — quietly. “With children who are nonswimmers, they [almost never] go through a distress phase where they call out and splash,” says Vittone. "They’ll go right into that bobbing instinctive drowning response.”

Arms out to the sides. Someone drowning rarely waves his arms above the surface. The drowning response directs him to spread his arms out to the sides and press down on the water in an attempt to keep the nose and mouth above the water.

Floating face down. Call for help if you see someone floating face down for more than a few seconds. If he is unconscious, a lifeguard will try to administer CPR and revive him.

Related: Inflatable Pools: Small Doesn’t Mean Safe

How to translate awareness of silent drowning into safety? Says Vittone, “The primary message is you have to realize that the involuntary drowning response is quiet. Small children can actually drown without having ever broken the surface of the water at all.”

How to prevent silent drowning (and any other drownings)

Vittone offers these tips to improve safety while swimming, especially if children are involved.

Give children your undivided attention. “You have to be looking at them to be watching them. Unless your eyes are on them, you’re not watching them. You can’t do anything else. You can’t look down at your email on your phone, you can’t text, you can’t look over and talk to somebody else and still be watching."

Get a substitute “water watcher. Constant watching is exhausting, so Vittone recommends that adults designate a “water watcher.” Take a card, watch for 15 minutes, and then hand the card off to the next water watcher. Suiting up your children in a Coast Guard-approved life jacket is also a wise idea, but it's no substitute for watching them.

Check in with the lifeguards.“ At open water things — oceans and lakes and rivers — talk to the lifeguards before you go in, because the lifeguards know the places that are dangerous to swim,” says Vittone. They’ll be able to steer you away from hazards such as jellyfish and rip currents that may not be visible, even to experienced swimmers. As far as swimming at unguarded beaches with children, Vittone has this to say: “Just don’t do it.”

Take quick action. At a pool, if you recognize that someone is drowning, you can take effective action even if you aren’t trained in rescue. Says Vittone, “If a 4-year-old is drowning in 3 feet of water, jump in and grab them. It’s over.” If the water is deeper, call the lifeguard and look for something that floats. “There’s always something that floats at the pool. Always. Push a seat cushion or flotation device toward the victim until they grab it and then they’re not drowning anymore.”

Related: Play It Safe at the Water Park

Alert lifeguards right away if you see someone in trouble. Says Vittone, people who are drowning sometimes “can’t even reach for something you throw to them. If you were to throw a rope to someone and it was 6 inches from them, they can’t stop drowning to reach over and grab it. Even if they did, if it doesn’t provide sufficient flotation to clear their airway, they’ll simply let go of it because it’s not of any help to them.”

If there are no lifeguards at a lake or beach and you’re a strong swimmer, you can attempt a rescue. Always swim out with a rescue buoy, though, because the victim’s first instinct is to climb on top of you. If there’s no buoy, take a towel so the drowning person can grasp it. If he can’t do that, approach him from behind so he can’t grab you around the neck.

The take-home message, says Vittone, is don’t expect drowning to look like something you’ve seen on the screen.

“It doesn’t look like what you think it is,” he says, stressing that you have to watch for cues. “It’s not swimming. It’s not playing. It’s not loud. It’s that other thing. It’s drowning.”

Greg Breining is a Minnesota-based journalist who writes about science, travel and nature for national and regional magazines, including Audubon and National Geographic Traveler. His books about the natural world include Wild Shore and Paddle North.