ATLANTIC CITY - Today shouldn't be all that bad a beach day - except for the menacing waves likely to keep lifeguards on their toes and swimmers in shallow water.

Hurricane Bill, energized by warm ocean temperatures, agitated the North Atlantic yesterday like a giant plunger, creating waves up to 28 feet near Bermuda. Officials said the waves could go as high as 47 feet in the open sea today.

"It's a very dangerous situation," said Hugh Cobb, marine forecaster for the National Hurricane Center in Miami.

Along the Atlantic coast, mariners will encounter waves of up to 12 feet off New Jersey and a daunting 20 feet off Cape Cod today and tonight, Cobb said.

Waves of up to eight feet are expected through tomorrow directly in front of lifeguard stands at the Jersey Shore, where riptides are all but a certainty, according to the National Weather Service office in Mount Holly.

Other than that, it should be a decent day at the Shore. No kidding. Heavy rains are expected inland, where a flash-flood watch has been posted, but along the coast highs are forecast to be in the mid-80s, with light winds and only a chance of showers.

If that's not enough, the water temperatures are hovering around 80 degrees - not that you're likely to be able to venture in deeper than your waist, if at all.

The weather will be "deceptively quiet," Cobb said.

Anticipating a heavy beach turnout, emergency management offices in Atlantic and Cape May Counties have warned municipalities and lifeguard patrols of potential widespread rip currents and dangerous surf conditions that could cause bathers to be knocked down and swept off by retreating waves.

Officials have not declared that no one should go into the water, but they have cautioned lifeguards to keep a close eye on swimmers.

"They're professionals. They'll use their discretion when it comes to letting people go into the water and making those kinds of decisions," said Frank McCall, director of Cape May County's Emergency Management Office.

"But we're warning the public that no one, under any circumstances, should venture into the water without a lifeguard present," McCall said.

That's not always easy for swimmers tempted by such warm water, especially with monstrous swells creating a surfer's paradise in places where waves are typically no more than five feet.

"Every surfer is an amateur meteorologist," said Frank Malatino, 24, who works at the Brighton Beach Surf Shop on Long Beach Island. "We just watch and wait for the kind of weather the experts are forecasting."

Today's weather will be far nastier inland, as a front mines some of the moisture that has thickened the atmosphere this week. Up to three inches of rain could fall in some places through tonight.

Though Bill will be far out to sea, terrorizing lobsters in the North Atlantic, the hurricane could play a role in the inland rains. Bill will cause the front to stall, making storms linger west of the Delaware River, said weather service meteorologist Greg Heavener.

Adding to the threat was what Heavener called a "pre-event," a big pulse of moisture far removed from the storm's center. Downpours well in advance of the storm were expected, with the entire region, except for the Shore counties, under a flash-flood watch through this morning.

"Our region is in a pretty good spot, atmospherically speaking, to have one of these pre-events," Heavener said. "You could be looking at a copious amount of rain."

It's not like bad weather can't happen to a good place. On Sept. 26 and 27, 1985, Philadelphia got hit with five inches of rain as a result of Hurricane Gloria, most of it falling with the storm several hundred miles away.

No hurricane has made landfall in the Garden State since 1903. Hurricanes in 1938 and 1944, the most damaging of the 20th Century, passed well offshore, but still managed to generate waves of 25 to 50 feet.

Nothing like that is expected to happen here this weekend.

"Even though it doesn't look like we're going to have anything more here than big waves and maybe some minor tidal flooding, we won't take our eyes off this thing until it passes well off New England," said Vincent Jones III, director of the Atlantic County Office of Emergency Preparedness.

"Our big concern right now," he said, "is that it's going to be a nice day and it's going to draw people into the warm water and cause injuries."

AtlantiCare Regional Medical Center in Atlantic City reports that it has treated a higher-than-usual number of water-related injuries - 237 in June and July - in its trauma center this summer. As of Thursday, about 20 patients had been admitted to the hospital with severe traumatic injuries since Memorial Day, about double the number during the same period last year.

Hospital officials attribute an increase in spinal-cord injuries this year to the growing popularity of surfing, body surfing and skim boarding, and to larger Shore crowds due to the warm water and lower gas prices, said Jennifer Tornetta, an AtlantiCare spokeswoman.

Weather forecasters have predicted that Bill, a major hurricane yesterday with maximum sustained winds at 105 m.p.h., will retain hurricane strength all the way to the Canadian Maritimes as it scoots between Bermuda and the U.S. Atlantic coast between today and Monday morning.

It should exploit the warm waters over the Gulf Stream and take advantage of above-normal ocean temperatures to the north of the Gulf Stream wall, said the hurricane center's Cobb.

The hurricane will likely work against itself, however, as it churns up cooler deep water to replace the warm water on ocean's surface. That will lower the amount of available energy.

"It will conspire in its own undoing," Cobb said.

Bill is reminiscent of Hurricane Gabrielle in 1989, which followed a similar path and incited 15- to 25-foot waves along the East Coast.

Gabrielle resulted in eight deaths, according to the National Oceanic and Atmospheric Administration, with some of the victims swept from jetties where they stood admiring the tall waves.

Contact staff writer Jacqueline L. Urgo at 609-823-9629 or jurgo@phillynews.com.