SANDUSKY, Ohio -- Judging by the quality of the hors d'oeuvres, not to mention the presence of two of the company's VIPs, it was an important day at Kalahari's water park in Sandusky, Ohio. Kalahari Resorts Chief Executive Officer Josef Haas and Chief Financial Officer Mary Bonte Spath were trying to persuade investors to part with more than half-a-million dollars each in exchange for a five-bed, three-bath, 1,500-square-foot condominium.

That's half-a-million dollars.

In Sandusky, Ohio.

For a condo not on the water, but at an indoor water park.

"It's an investment opportunity," Ms. Spath said. Buyers get to use the condo a couple weekends a year. They enter a rental agreement with Kalahari, which will market the condo and its three separate rooms as hotel suites, available to the thousands of families, business travelers and conference-goers who visit the water park each year.

The buyer gets to keep about 55 percent of the room revenue. The water park, meanwhile, gets the upfront cash from the investors, and uses that to keep down its own debt levels as it expands across 140 acres, adding a new hotel tower and more than doubling the size of its current water park, from the present 80,000 square feet to more than 170,000, to be completed by December.

With wave pools, surfing rides, slides, adult hot tubs, a swim-up bar and more, all enclosed in a giant box and set to about 85 degrees Fahrenheit, it will be the biggest indoor water park in the United States -- for now.

Indoor water parks -- think Sandcastle, miniaturized and with a roof on it -- are the biggest thing to hit the hotel and hospitality industry in a generation. At the beginning of 2000, there were 18 indoor water park hotels in America. By 2002, there were 50, with half of them in Wisconsin Dells, Wisc., the birthplace of the indoor, year-round water park. By the end of this year, there will be 184, and dozens more are in the planning stages or under construction.

By 2009, Carmen Shick, the New Castle developer whose acres along Route 422 will be the site of a Lawrence County racetrack and casino, may join the ranks. On a plot adjacent to the racetrack, he says he has designs on a hotel-and-condo development that would be anchored by a giant indoor water park, similar to Kalahari's model.

His would be 150,000 square feet -- about three football fields, minus the end zones -- and would be on par with other mega-size aquatic playgrounds, almost twice as big as the popular Splash Lagoon, in Erie.

"I think what hooked me was friends of mine that had taken their kids to Splash Lagoon," Mr. Shick said. "I tried to make a reservation there, and you had to make reservations three months ahead of time. ... No first-rate resort should be without one."

These days, property developers in the Northeast and Midwest view indoor water parks as the must-have accessory to any well-regarded hotel or resort. Builders see visions of dancing dollar signs, hoping the park will be the tourist driver that brings in visitors and families year-round, allowing them to charge elevated room rates even after the summer vacation season ends.

Mr. Shick's would be the first one to be built in southwestern Pennsylvania, but it's not the first one to be discussed here.

Craig Cozza, the developer whose Cozza Enterprises LLP owns Monroeville's Palace Inn, says he wants to add an indoor water park to the hotel if he's able to talk the Pennsylvania Gaming Control Board into letting him run a small casino out of the hotel. Both Seven Springs Mountain Resort in Somerset County and Nemacolin Woodlands in Fayette have talked about adding indoor water parks to their existing stable of attractions. Beaver County developer Bob Svihra says he wants to build an indoor water park at the junction of Route 60 and the Pennsylvania Turnpike. Kennywood, which owns Sandcastle water park, has discussed building an indoor water park and hotel, if the north branch of the Mon-Fayette Expressway were ever completed.

"It would be nice to bring in income the rest of the year," said Mary Lou Rosemeyer, Kennywood's PR director. "If it's raining, it's still works."

American indoor water parks got their start in 1994, when Wisconsin Dells hotel owner Stan Anderson added some doodads to his indoor pool, and occupancy rates skyrocketed. What started out as a regional oddity soon became an industry trend -- the 184 indoor water park hotels will have 31,400 rooms, connected to a total of 4.4 million square feet of splishy, splashy fun.

They come in all shapes, sizes and themes. Some, like Kalahari and the popular Great Wolf Lodge chain, are built ground up, with huge parks integrated in the hotel space. Existing hotels, trying to cash in, are retrofitting their properties to include smaller water parks -- it's called "add-a-box" within the industry. Generally these parks are less than 50,000 square feet.

The more lavish parks are expensive to build, at $350 to $450 per square feet, including elaborate ventilation and underground plumbing. They are also expensive to maintain.

As a result, by themselves they wouldn't be profitable.

"You couldn't just build a 50,000-square-foot box and charge admission," said Bill Haralson, an indoor water park and hospitality consultant. "It's almost a loss-leader."

The money comes when the hotel side of the operation is able to charge an extra $25 or more per head, per night. And since the typical party visiting a water park hotel is a larger one, about five people, the hotel can make $125 extra or more on its rooms. (Some parks allow paid outside visitors, but most restrict water park use to hotel guests.)

So far, indoor water parks have gravitated to smaller towns with built-in tourist traffic -- the Poconos, Niagara Falls, Williamsburg, Va., and Sandusky, a town whose service economy is largely based on the summertime Cedar Point visitors. But Kalahari and its smaller cousins (Great Wolf and Cedar Point's Castaway Bay and Rain, attached to a Quality Inn & Suites) have reshaped the previously rural area south of the city.

Just six years ago, "it was vacant ground," said Mark Litten, head of the Greater Erie County Marketing Group, based in Sandusky. Now, a fifth indoor water park, Maui Sands, is under construction and set to open in 2008, while a sixth, Coyote Falls, was looking at Sandusky, but was lured to Rhode Island.

Why so popular? In Mr. Litten's eyes, indoor water parks are the public pools of the future.

"They're gone" he said of the days when every borough had a public pool open during the summer. "They were money-losers."

Not that an indoor water park is a guaranteed mint. Great Wolf continues to expand amid a net loss of $1.7 million in the second quarter of 2007, 5 cents a share, compared with a net loss of $1.4 million in the same quarter in 2006.

That's why it helps to have a nearby attraction like an amusement park, a ski resort, a natural wonder -- or even a casino.

"If you depend on the water park market, you're probably not going to run 50 percent occupancy," Mr. Haralson said. "I think an indoor water park is a great engine for driving room sales, but it has its limitations."