PHILADELPHIA - A survivor of the collision between a barge and a duck boat in the Delaware River that dumped dozens of tourists into the water, leaving two Hungarian language students missing, called the experience "harrowing" and "surreal" Thursday.

Hope faded for finding the two passengers alive Thursday; officials said they were a 16-year-old girl and 20-year-old man. A search for them continued, but visibility in the 50-foot-deep water was too low to send divers in.

The boat had no history of mechanical problems before it caught fire, said Chris Herschen, president of Ride the Ducks, the Norcross, Ga.-based company that owns it. The captain appears to have followed all proper procedures during the emergency, he said at a news conference.

It started out as just an inconvenience when smoke started to roll out of the boat's engine as it entered the water, Sandy Cohen, 67, told The Associated Press in a phone interview Thursday from her home in Durham, N.C.

The tour guide said a tug boat would be on its way to carry passengers back to shore, she recalled. She was on the phone with her husband to let him know she'd be late, but the call ended abruptly as other passengers screamed.

"Someone said, 'Oh my God, there's a barge coming, and it doesn't look like it's stopping,'" she said.

She grabbed for a lifejacket from a hook above her seat as the boat was struck and started to sink. She was quickly underwater, grabbing the jacket with one hand as her feet tangled up with those of others.

When she surfaced, she said, she realized one of the Hungarian teenagers on the boat was also hanging onto the same flotation device.

"I just told her, 'Don't let go,' and made sure we both stayed calm," she said.

They were rescued five to 10 minutes later.

While crews searched for the missing, the tour company, Norcross, Ga.-based Ride the Ducks, said Thursday that it was suspending operations nationwide, a day after it suspended its Philadelphia tours. It also operates tours in San Francisco, Atlanta, Newport, Ky., and Branson, Mo. A Ride the Ducks operation in Seattle is independently owned and remained open for business Thursday.

"Our thoughts and prayers are with our Philadelphia tour guests, crew members and their families," the company said in a statement. "We are attending to their needs first. In the interim, we have voluntarily suspended our Ride The Ducks operations nationwide."

Visibility at the bottom of the murky Delaware River was nil, said Philadelphia police Lt. Andrew Napoli, speaking of his earlier dives.

"The vehicle is laying upright on its wheels," he said. "There could be bodies inside, we're not sure. ... With the currents being what they are, if it went down with bodies inside, the bodies could very well have been washed out of the vessel."

The 37 people aboard the six-wheeled duck boat were tossed overboard when the tugboat-pushed barge hit it after it had been adrift for a few minutes, police said. Most were plucked from the river by other vessels in a frantic rescue operation that happened in full view of Penn's Landing, just south of the massive Ben Franklin Bridge connecting Philadelphia to New Jersey.

The duck boat, which can travel seamlessly on land and water, had driven into the river Wednesday afternoon and suffered a mechanical problem and the small fire, officials said. It was struck about 10 minutes later.

Ten people were taken to a hospital; two declined treatment, and eight were treated and released, Hahnemann University Hospital spokeswoman Coleen Cannon said.

The National Transportation Safety Board said it planned to try to obtain any radio recordings, any possible mayday calls, photographs from witnesses or people aboard and other evidence as its investigators remain in Philadelphia over the next several days.

Investigators planned to try to figure out why the vessels collided and "how conspicuous would that duck have been" to the tugboat pushing the 250-foot-long barge, NTSB member Robert Sumwalt said. NTSB officials also hoped to conduct witness interviews, he said.

The company hoped to raise the boat to the surface soon, said Herschen, the company president.

Ride the Ducks has been in Philadelphia since 2003. Passengers board the duck boats at the Independence Mall and are driven on a tour of the Old City neighborhood. Afterward they ride into the Delaware River from a ramp south of the Ben Franklin Bridge.

As of 2000, there were more than 250 refurbished amphibious vehicles in service nationwide among various operators, according to the NTSB.

Some duck boats are amphibious military personnel carriers dating to World War II that have been restored and reconditioned for peacetime use. Known by their original military acronym as DUKWs, they were first introduced in the tourism market in 1946 in the Wisconsin Dells, where about 120 of the vessels now operate.

A duck boat sank at Hot Springs, Ark., in May 1999, killing 13 of the 21 people aboard after its bilge pump failed. The NTSB blamed inadequate maintenance and recommended that duck boats have backup flotation devices.

In June 2002, four people were killed when an amphibious tour boat, the Lady Duck, sank in the Ottawa River near Canada's Parliament.