Rob Pace is excited to go swimming with his two kids on their upcoming vacation in Palm Beach, Fla. One problem: The 39-year-old never learned to swim.

So twice a week before work, Mr. Pace sneaks out of the house for adult swim lessons at a pool in Manhattan's Financial District. After three weeks, he can now hold his breath under water and glide while kicking for about a lap. He is waiting until his vacation to surprise his kids with his new skills.

'They're going to be absolutely stunned,' says Mr. Pace, who lives in East Meadow, N.Y., and is a controller at a real-estate company.

An estimated 37% of U.S. adults can't swim 24 yards, the length of a typical recreation-center pool, according to the Centers for Disease Control and Prevention. Adults -- including those who are able to swim -- make up more than 70% of drowning deaths in the U.S. each year, according to the CDC.

Adults may miss out on learning to swim if they come from a culture where swimming isn't widely popular, or they grow up in metropolitan areas without easy pool access. Others are simply afraid -- a fear sometimes fostered by overanxious parents or a terrifying incident early in life. Teaching late learners tends to take longer and requires different techniques than those used with children.

A growing number of swim clinics are specializing in adult lessons, partlybecause learning to swim later in life can be a little embarrassing.

Beth Davis, who operates her own swim clinic in Boulder, Colo., says the number of adults in her practice has nearly doubled in recent years, from 34 adult students in 2009 to 67 currently. Lori Pailet, a founder and director of AquaSkills, the Manhattan facility, says her adult clientele typically increases by 15% to 25% each year. Lessons there cost $100 to register and $1,200 for 10 private lessons.

Mr. Pace, who is taking lessons at AquaSkills, says his parents never knew how to swim and raised him in New York City, where he didn't have easy access to a pool. 'Most of my friends at the time didn't know how to swim, so it didn't seem like a big deal,' he says.

Joseph Riggio, who lives in Brooklyn, also signed up for swim lessons at AquaSkills, where his biggest hurdle has been overcoming an intense fear of the water. He says his father believed the best way to protect his son was to make him afraid of the water. Mr. Riggio recalls a trip to Coney Island when he was about 7 years old in which his father pushed him under the water repeatedly.

'After he threw me into the water the third time, I stayed down, and then he pulled me up. I was choking and coughing up water,' says Mr. Riggio, owner of New York Pizza Suprema, a restaurant in Manhattan.

From then on, learning to swim became a problem for Mr. Riggio. He tried taking lessons as both a child and adult but couldn't overcome his phobia.

To ease Mr. Riggio's fear, Ms. Pailet spent the first few lessons simply talking and walking through the water with him.

'She asked about my family and children and teased me about my pizza business as we did exercises,' he says.

Mr. Riggio had to unlearn all of his poor habits and commit proper technique to muscle memory. Now, he can do the arms, legs and breathing motions separately but has trouble syncing it all up. When he tries, often he just ends up splashing.

'Yes, it's embarrassing, but I'm going to be more embarrassed if I'm not a good swimmer but my children are,' Mr. Riggio says.

The process of learning to swim can be slow and frustrating for adults who, unlike children, are 'very results focused,' says John Fitzpatrick, owner and head coach of a swim facility, Chicago Blue Dolphins.

Mr. Fitzpatrick first teaches adults how to float and glide by kicking off the side of the pool. 'These activities -- while fundamental and critical to them being successful -- do not seem to them like they're swimming,' he says. As a result, the adults tend to get frustrated with their seemingly slow progress.

In Manhattan, Ms. Pailet says one client wanted to skip the breathing and buoyancy basics and just 'get it over with.' At one point, she says, he became so frustrated, he began smacking the water with a foam noodle.

While learning the techniques are important, Ms. Davis, the Boulder instructor, also teaches adults how to play. 'Children don't need to be taught how to play, but adults are different creatures,' she says. 'They don't know how to respond to the water. It's this strange murky thing.'

In a game called 'bottom to bottom,' students jump up in the water, then let themselves fall bottom-first to the floor of the pool. Sometimes in group lessons she will have adults compete in relays or swim in pairs, with one person doing the arm stroke while the other grips his partner's ankles and kicks. Ms. Davis calls her methods 'functional play.'

'Adults work all day. I don't want them to view swimming in the same light,' she says.

Frank Papania, who lives in Bellmore, N.Y., signed up one week after his co-worker, Mr. Pace. Now the two, who have been friends for 17 years, take lessons together.

'We encourage each other more than anything else, but we will give helpful hints when we can,' says Mr. Papania, 40. 'We'll say things like, 'Hey, straighten your legs.' '

The goal is to achieve 'water competency,' defined by the American Red Cross Scientific Advisory Council as being able to enter and exit the water, turn 360 degrees afloat, tread or float for one minute and move through the water for at least 25 yards.

Jess White, who grew up landlocked in Kansas and now lives in Boulder, says she never felt confident in the water. In 2010, she signed up for lessons with Ms. Davis to become more comfortable and ultimatelycompete in triathlons.

Learning to swim took longer than she thought, about five months. She didn't tell many people at first, hesitant to admit 'learning what seems like a childlike skill as an adult.' But her efforts show. Last summer, she swam 500 meters in a triathlon and plans to swim 1,500 meters in one this month. Ms. White, 27, is still taking lessons to refine her stroke.

'Pretty much everybody knows now,' she says. 'It became a point of pride for me when I could say, 'Look at me, I can swim.' '