A sightseeing helicopter carrying a family of four crashed just offshore of the USS Arizona Memorial visitor center Thursday morning, shocking tourists and prompting a few people to dive into Pearl Harbor to help those in the overturned aircraft.

A 16-year-old boy remained in critical condition Thursday afternoon at Pali Momi Medical Center, the hospital said. The Navy said the crashed helicopter apparently belonged to Genesis Helicopters.

Amateur video shows the Bell 206 helicopter, with five people aboard, heading toward shore and attempting to maintain level flight at an altitude of about 50 feet before pancaking into the nearshore waters, its blades churning sea spray before the aircraft quickly rolls over.

“Holy crap!” someone nearby can be heard exclaiming on a recording.

Three of the family members, who might have been from Canada, were brought to Pali Momi Medical Center. One was in critical condition, and two were in stable condition late Thursday afternoon, the hospital said. A fourth family member was treated at another hospital and released. The fifth person on board was taken to Tripler Army Medical Center, an official said.

At about 10:30 a.m. Honolulu Emergency Medical Services responded to the crash and transported the 16-year-old in very critical condition. A 50-year-old man and 45-year-old woman were treated and transported in stable condition.

Chris Gardner, a guide with Keawe Adventures, heard the chopper crash, ran around the theater on the grounds, then jumped into the water to help with the stricken craft, which was in about 10 feet of water just 15 to 20 feet from shore.

The teen was stuck inside the starboard rear seat, “so I jumped in and I tried to extract him,” Gardner said. “The other four had been extracted already, or they got out on their own.”

Gardner said he and a federal police officer named Brian took turns diving down and trying to cut the unconscious teen out of the four-point harness.

“It was so murky under there,” Gardner said. He said the officer finally got the boy out. Meanwhile a few people were bobbing in the water, and rescuers in rigid-hull inflatable boats were coming. Gardner said it was “just instinct” that prompted him to help.

Brett and Lorraine Palmer, visiting from Australia, didn’t see the crash, but heard it. “Then everyone was yelling and screaming and running across the lawn to where (the helicopter) had gone in. It only just missed the land,” Brett Palmer said.

Five to six people were in the water, some with orange inflatable vests. The Palmers also saw two men diving down to the helicopter.

“We walked away, and then the crowd cheered and clapped so clearly, they did get somebody out,” Brett Palmer said. He added that there were some visitors stuck for a while on the memorial itself while the crash was sorted out.

“It’s a very sad thing to see,” Lorraine Palmer said.

The first responders included Navy boats from Joint Base Pearl Harbor-Hickam and federal firefighters, according to the Navy.

Amber Moncrieff, also a tourist from Australia, said she was waiting in line for the 10:45 a.m. showing of the movie at the National Park Service’s visitor center theater when she heard two loud bangs. Another member of her group walked around the theater and saw that a helicopter had crashed.

“There were a lot of young men coming out of the water wet,” Moncrieff said. The vision of the rescuers stuck in her mind. “When something bad happens, so many people will come out and help,” she said.

Moncrieff said she saw at least four people being taken away on stretchers. Paramedics were administering CPR to one victim, she said.

The National Park Service and the Navy closed the USS Arizona Memorial Visitor Center shortly after the crash. Police blocked the entrance to the visitor center and didn’t allow anyone in, and stunned visitors slowly made their way out.

Glenn Powers, visiting from Rhode Island with his wife, Susan, never made it in to see the memorial.

“We’ve been here a week, and today was the day we had reservations,” Powers said. Powers said he was “disappointed. But it is what it is.”

The company the Navy said operated the helicopter, Genesis Helicopters, features the Hawaii News Now helicopter on its website.

It advertises “Doors Off” helicopter tours, and the company’s website features a prominent aerial photo of the USS Arizona Memorial and USS Missouri in Pearl Harbor.

A check of the tail number showed it to be a 1979 Bell 206B helicopter, known as a Jet Ranger. According to FAA records, the Bell 206B is owned by Jeffrey Gebhard of Kailua, who owns Genesis Helicopters.

The helicopter has a valid certificate that was issued in August. It can carry four passengers and a pilot, according to the Genesis Helicopters website. Gebhard said on the website that he started Genesis in 1999. He could not be reached immediately for comment.

The Arizona Memorial Visitor Center will be open today, but the film that depicts events leading to the Pearl Harbor attack and boat tours to the memorial are suspended because of the crash investigation. The Park Service said normal operations would resume at other Pearl Harbor historic sites: the Battleship Missouri Memorial, the USS Bowfin Submarine Museum and the Pacific Aviation Museum Pearl Harbor.

Federal Aviation Administration investigators were en route, and the National Transportation Safety Board, which investigates aviation crashes, was notified, said Lynn Lunsford, an FAA spokeswoman.

According to the NTSB, there were 17 other civilian helicopter accidents in Hawaii involving “substantial” damage between July 2010 and Thursday’s crash. The most recent, on Jan. 17, involved an Airbus EC130 that landed hard on the beach two miles west of Hanalei, Kauai, after a reported loss of engine power.

The commercial pilot and two passengers on the Blue Hawaiian Helicopters aircraft sustained minor injuries, and four passengers were seriously injured, the NTSB said. The helicopter sustained substantial damage to the “tailboom” and airframe.

Sixteen of the 17 accidents were nonfatal, and nine involved small Robinson helicopters, according to the reporting.

Honolulu Star-Advertiser reporters Craig Gima, Dan Nakaso, William Cole and Gregg K. Kakesako contributed to this report.