SUP, as it's called for short, looks exactly as it sounds: you stand on a large surfboard and propel yourself forward with a paddle. But, unlike traditional surfing, you don't have to wait for the waves. In fact, SUP, which is wildly popular, can be done on lakes, rivers, pools or any sufficiently large body of water. "It's completely blown up in the past five years and every spring it just blows up even more," says Jim Brewer, 45, a painting contractor who, in October 2008 and in spite of everyone calling him nuts, opened Blueline Stand-Up Paddle Surf in Santa Barbara, Calif., the first fully dedicated SUP shop in the country. "We thought it was phasing out, but then we realized that it's just beginning. It's going mainstream." (See pictures of surfing wipeouts at LIFE.com.)

Three weeks ago, for instance, while regular surf shops around the country were struggling to stay afloat, Brewer's store sold 16 boards - which start at about $1,500 - in one day. "If I had opened a surf shop eight months ago, we would have been out of business right now, no doubt," he says. Instead, Brewer, who also works as a distributor, fields calls for paddleboards from kayak and surf shops all over the country. "They know that's the only thing they can sell right now," says Brewer, who compares the sport's skyrocketing trajectory to snowboarding, which similarly gained traction in the 1980s and '90s. "A lot of people are using it to help save their business." (Read about how recession is threatening the original Surf City.)

It's generally agreed that the sport has roots in ancient Polynesia, but it didn't really enter the modern mindset until the mid 20th-century, when Waikiki's "beach boys" decided to stand up on their longboards and paddle around with outrigger canoe oars to get a better look at their surfing students, spot far-off waves, take photos for tourists or simply to have something to do on flat days. It wasn't until the late 1990s that the modern explosion began, thanks to big wave surfer and exercise guru Laird Hamilton picking up SUP and publicizing it as simultaneously adventurous, peaceful and a solid form of core conditioning for surfers and non-surfers alike. (Read an interview with surfing legend Kelly Slater.)

Since then, it's attracted everyone from the "little old lady to the hardcore guys," says Brewer, and become the new favorite sport of celebrities - Julia Roberts recently bought a board at Brewer's shop, joining the paddling ranks of Kate Hudson, Jennifer Aniston, Matthew McConaughey and Lance Armstrong, among others. It's already spawned new manufacturing: the SUP boards are specially designed, longer and wider than traditional boards. Meanwhile, multiple SUP magazines are now being published, races and wave-riding contests are popping up every month, and, as with any new-wave trend, a whole slew of entrepreneurs are trying to cash in, selling everything from boards and paddles to board bags, car racks, and specialized clothing lines.

But while SUP enthusiasts have became a daily dot on the horizon of many a coastal California city, it's the market for lakes and rivers that has everyone really excited, says Oahu native and former carpenter Blane Chambers, 45, whose company Paddle Surf Hawaii was one of the world's first major makers and distributors of paddleboards. "The flat-water market is just growing everyday," says Chambers in his Hawaiian drawl, explaining that his sales rep in Minnesota is "so excited" after doing the rounds at kayak shops in that lake-filled state. "It's crazy how fast this thing is growing. It's in France, Australia, Brazil - anywhere there's water, it's starting."

When Paddle Surf Hawaii started in July 2006, Chambers would sell two or three boards out of his garage each month. Today, after his business grew 900% between 2007 and 2008, the boards are shipped by the container load. Chambers sells about 1,000 per month, including more than 150 a month out of his central Oahu shop. Chambers, who lost 40 lbs from stand-up paddle surfing, says, "We can't expand fast enough. We can't even supply everybody." (See pictures of a preppy summer vacation at LIFE.com.)

One SUP neophyte who did manage to get his hands on a new board is Mike Zapata, 34, the director of sales at a Santa Barbara music technology company, who lives three blocks from the beach. "The problem with surfing is that it's so inconsistent, and I don't have a lot of time," he says. "I needed something that I could count on more." So he bought a paddleboard, and now fits the workout into his daily routine three times a week. "It's been awesome. I really enjoy it," says Zapata, who's lost a couple pounds in just a few weeks. "It's the perfect amount of time to escape and get a little activity in and recharge, and I feel it in places that I don't when playing basketball or doing other activities."

Traditional surfers, meanwhile, aren't always so stoked about the newcomer sport and its practitioners. They see SUPpers as more competition on already overcrowded swells. And many of the the newbies never learned wave-riding etiquette, which involves waiting for your turn and not cutting other surfers off. But Brewer, who grew up surfing and appreciates the concern, says such generalizing is foolish. "We have this saying, 'A kook is a kook,'" he explains. "If he's out there being an idiot on a paddleboard, he's also an idiot on a surfboard."

Politics aside, the foreseeable future looks pretty bright for those in front of the SUP wave. Says Chambers, "When I started the company, it was pretty obvious that it had the potential to be bigger than windsurfing. Then I thought, 'This is going to be as big as surfing.' And now I think it's going to be bigger than windsurfing, kitesurfing, surfing - everything put together - because it can be done anywhere."