This is not your typical pool party.

This is open-water swimming, and it is challenging.

On Saturday, at Deer Creek Reservoir in Provo Canyon, dozens of Utah's strongest swimmers will hop into the water and, stroke after stroke, swim as far as 10 miles.

Others will go 10 kilometers, some five kilometers and others just a mile.

All will be part of a growing family of endurance athletes who push themselves to the limits in ways few others do.

"They kind of think you're crazy," said Gordon Gridley, a Bountiful resident who will plunge into the water for the 10-mile swim on Saturday, referring to the reaction he gets when he explains his addiction to open water.

"They just can't believe it."

Considering most adults rarely swim farther than from the diving board at the community pool to the side to get out, the idea of swimming mile after mile does sound a little crazy. But according to those who love the sport, it is remarkably easy — once you dive into it.

"It's a lot more fun than swimming in the pool," said Salt Lake's John Karren, who will test himself Saturday in the 10K division. "If you get into a good rhythm, you really can go as long as you want to."

For some, the sport is a part of their lives because they are triathletes. As the first part of most triathlons, swimming long distances is just part of the training. But training in three different sports for several hours per week can cause one to gravitate to one particular sport — swimming, easily the lowest-impact part of a triathlon — has become the focus for many.

"It's relaxing," Gridley said. "For me, swimming long distances is just like going for a long walk. I get started, settle in, and I could swim all day."

The Deer Creek Open Water Marathon won't be all day, though. Even those braving the water for 10 miles will probably finish in four or five hours. With the aid of a support vehicle, usually a friend in a kayak carrying the drinking water and nutrition an endurance athlete needs, the swimmers will leave shore and are not allowed other assistance until they either finish the race or give up.

Swimming in a large body of water is much different than putting in several dozen laps at the recreation center. Winds create waves, gravity creates currents and an unexpected storm can create a deadly environment.

Last year's event was stopped, with most swimmers still in the water, after a late-summer storm rolled in and sent rain and hail onto the swimmers and their support crews as the temperature plummeted into the 40s.

"I was about 100 yards from the finish," Karren said, "when the state park guy came up to me and told me to get out of the water. I looked up and saw the finish and said, 'I'm not getting out now.' The water was still about 70 degrees, but our support in the kayaks were getting it pretty bad. There were 4-foot swells, and I only saw my support once, and that was at the turnaround."

Gridley, racing in the 10-mile event last year, had to leave the water and retreat to the finish line disappointed.

This year, weather forecasts look much better, and the field is growing.

"I think it doubles ever year," Gridley said of the four-year-old event. "We get people from all over, not just from Utah."

Gridley is one of the tough guys of the sport. He swims without the assistance of a wetsuit, and his distances are longer than most. Earlier this month, he and a friend swam from Antelope Island to Fremont Island, spent the night camping, then swam back in the morning. He has reserved a spot in 2012 to swim the British Channel.

To qualify for that swim — not anyone can just show up and swim it — Gridley is training with frequent open-water swims and is even acclimating his body by taking cold baths instead of hot showers. Next year, he plans on making a trip to Bear Lake, where he hopes to become one of the few people ever to have swum from one end to the other — a 19-mile effort.

James Jonnson did just that a couple of weeks ago when he started at the lake's southernmost point and didn't stop until he had reached the northernmost point eight hours and 53 minutes later ?— crushing the record of 15 hours, set by John Quackenbush in 2009. Neal Phelps, who did it with a wetsuit and flippers, is the only other person to have a recognized tip-to-tip Bear Lake swim.

No one will be swimming that far this weekend at Deer Creek, and for most, the clock is not really a concern.

"I'd guess that 80 percent are probably just seeing what they can do," Karren said. "That's what I did the first time."

Gridley is worried most about the finish line rather than the finish time.

"For me, with open-water swimming, it's more about the distance," he said. "I'm interested in seeing how far I can go."

Karren, typically one of the fastest swimmers at any triathlon he enters, is hoping to finish among the top three at his distance.