WaveOne gives swimmers practice jumping into the water at National Harbor, above, and then a chance to work on endurance and technique as they round buoys set up off the North Pier, below.

Sick of swimming pools? Try a much bigger body of water: the Potomac River.

The Concept

“It’s a different element to not have anything around you, to be completely engulfed in nature,” says Denis Crean, who has been racing in oceans, rivers and lakes for the past 15 years.

He was frustrated that there were no places close to home where he could train for those races. So he created WaveOne Swimming (waveoneswimming.com), which — for the past three summers — has been helping Washingtonians sample open-water swims Thursday nights at National Harbor.

The Atmosphere

Everyone wants to know if the Potomac is clean enough for swimming. That includes Crean and Potomac Valley Swimming Safety Committee Chairman Kurt Thiel, who continuously monitor the water and cancel swims if there’s a risk of contamination. (Bacteria levels fluctuate with rainfall, and it’s not always safe to dive in.)

“No one here has ever had any issues,” Crean says. “That’s probably because if we don’t think it’s right, we cancel. We’re not going to put people’s health in jeopardy.”

That rule also applies to bad weather, which is why WaveOne calls off swims if there’s a threat of dangerous storms or poor visibility. But part of the thrill of open-water swimming is facing whatever challenges Mother Nature delivers, be it floating branches or choppy water. And there’s always at least one certified lifeguard in a kayak, as well as a handful of spotters in boats and on boards in the water, in case of emergency.

“As long as the condition’s safe, we’re gonna go, because we want people to experience wind, currents and all those things,” Crean says. “When they get into a race, they need to be prepared.”

The Logistics

The WaveOne crew uses buoys to set up a 300- to 400-meter course off National Harbor’s North Pier. People start arriving about 30 minutes prior to the announced swim time to check in, socialize and listen to safety instructions.

One note Thiel emphasizes is how swimmers can let him know they’re OK when he approaches in his kayak. They’re taught to put a hand on their swim cap and hold it there for three seconds. (“If I even have a hint that you’re distressed or in distress, you’ll probably be in a rescue tube and floating on your back in a hurry,” Thiel says.)

After the instructions, everyone jumps into the water from the dock.

Most swimmers head out on their own. For those who like a more structured swim, coach Tammy Lowengrub often leads drills, such as alternating between easy and fast laps. Sometimes she has the athletes practice race starts to get used to being knocked around by other swimmers.

“We’ll get them bunched together and blow a whistle, and then they’ll take off,” Crean says.

The Workout

Open-water swimming is always tougher than laps in a pool, says triathlete and nuclear engineer Tom Hull, who lives in Arlington.

“You’re burning more calories, you’re getting better as a swimmer, you’re building strength,” says the 51-year-old, who notes that the challenge can be as much mental as it is physical. “The idea of black water, cold water, unknown, swimming in the dark, you see nothing. That’s scary stuff.” Now in his third year of training with WaveOne, Hull says he’s overcome that fear and worked his way up to a 6.24-mile swim.

The Crowd

An average of 70 triathletes, competitive swimmers and Special Olympians come out to splash around the Potomac on Thursday nights. But there aren’t any beginners: To participate, you must be able to cover 750 meters in a pool or 400 to 500 meters in the open water without stopping.

“Within 10, 20 yards we know if you cannot do it,” says Crean, who will pull novices out of the water if necessary. “We don’t want to divert our resources to one person.” (Not ready? WaveOne offers pool clinics throughout the year to help wannabe open-water swimmers get in shape.)

After spending time together in the drink, it’s time for some drinks. Everyone dries off and heads to a nearby bar once the sun goes down.

Plop Psychology: Safety officer Kurt Thiel has advice for alleviating the fear of taking the plunge: “Get your faces in the water after you get in. It starts the mammalian diving reflex. Your pulse rate will come down, your breathing rate will come under control, and your chest will not be cramping up.”

Through Sept. 26; Thursdays, 75 minutes before sunset at National Harbor North Pier (nearest to the carousel); $25 in advance; $30 on-site. Go to waveoneswimming.com for details.