In April, the new Gansevoort South Hotel in Miami claimed the title of the country's largest rooftop pool (110 feet long); it also has an infinity edge a few floors down for good measure. At 11 a.m. every day in Nevis, the Four Seasons Resort fans out its spa staff to offer services poolside. On Memorial Day weekend, the Viceroy Santa Monica will add a pool concierge and, in a couple of its poolside cabanas, movies. The Ritz-Carlton South Beach, which opened in 2004, offers a rolling mojito cart and staffers to polish guests' sunglasses. And at the new Trisara resort in Phuket, Thailand, every suite has a private pool.   

Fueled by unprecedented competition for guests, hotels are pouring millions of dollars into pools—not bathtub-size afterthoughts, but sybaritic, resort-style pool complexes and waterside amenities (see related slideshow). These new ersatz beach clubs go beyond the well-stocked cabana to add acres of real estate, residential design and furniture, gadgets, and increased staffing. But the news isn't all good, especially for travelers who like kids—or quiet.

Hotel construction and the industry's inventory of new rooms has been climbing for 17 straight quarters, according to New Hampshire-based research firm Lodging Econometrics, and there's a record number of rooms in the pipeline. At the end of the first quarter of 2008, there were 5,007 hotel-construction projects under way in the U.S., representing 779,307 new rooms. In that climate, and with more rivals on the horizon, hoteliers say they're using lush pools and amenities to set themselves apart.

"[Guests are] spending more time by the pool than in their rooms," says Mark Tamis, regional vice president of Morgans Hotel Group's Southeastern properties. Now that the business traveler can "surf [the Web] surfside," without the boss knowing, some are simply parking all day by the pool. The company, whose portfolio includes the Delano in Miami and the Mondrian in Los Angeles, has started ratcheting up its pool amenities, changing layout, furniture, space, and bar-menu items.

Indeed, hotel executives say a number of factors have come together—wireless internet, global warming, the popularity of "outdoor rooms" in people's backyards—to create a perfect splash of pool construction and demand. Developers of condo hotels are feeding the trend as they add pools to try to boost sales.

But the main reason hotels are building pools is that they are proving so profitable, now generating nearly half as much revenue as the industry's biggest cash cow, room service. Hotel guests spent about $1.9 billion on food and beverages at hotel pools (and snack bars, coffee carts, and golf clubs—but none of those amenities received the focus pools have) in 2005, up 3.5 percent from the year earlier, according to a study by the University of Delaware's School of Hotel, Restaurant & Institutional Management, and that revenue stream has been growing.