A Canadian family of four visiting Hawaii survived a dramatic helicopter crash in Pearl Harbor, but three are in hospital with injuries.

A 15-year-old boy who was trapped underwater and had to be cut free from his seat is in critical condition.

A 45-year-old woman and a 50-year-old man are in stable condition. A fourth family member was treated and released Thursday. No names were released.

Details on the pilot, who remained hospitalized Friday, were not immediately available.

Tourist Shawn Winrich was taking photos of Pearl Harbor when he saw a helicopter "essentially coming straight at us" at the popular tourist destination.

He switched to video, recording the helicopter's dramatic drop into the water below.

Hawaii Helicopter Crash

In this image taken from video shot by tourist Shawn Winrich, a helicopter can be seen crashing near Pearl Harbor, Hawaii, on Thursday. (Associated Press)

"All of a sudden it essentially just fell out of the sky and crash-landed in the water," the Madison, Wis., man said.

He stopped filming and jumped in the water to help.

U.S. Navy spokeswoman Agnes Tauyan said the helicopter sank shortly after going down near the visitor centre Thursday.

'I took off my shirt and dove in'

Another bystander who jumped in to help was tour guide Chris Gardner, who was with a group at the Pearl Harbor Visitor Centre when he heard the crash.

"I took off my shirt and dove in," he said, describing how he, a navy sailor, a federal police officer and another man took turns diving to the submerged helicopter and trying to free a passenger with a knife. "He was strapped into his seat in the back of the aircraft."

Honolulu paramedics treated the 15-year-old and took him to a hospital, said Shayne Enright, a spokeswoman for the Honolulu Emergency Services Department.

"It was a team effort and we pray that he's OK," Gardner said of the teen he helped free.

Jumping in to help is part of the instinct growing up in Hawaii, he said. His eyes were still burning from the fuel in the water, he said.

Hawaii Helicopter Crash

A family of four from Canada who were passengers the helicopter survived the crash. (Shawn Winrich/Associated Press)

Winrich's daughter, Justice, said she watched as the helicopter plopped down into the water and saw three people get out immediately and swim to shore.

"It was crazy," she said. "You go on vacation and you never think you're going to see something like that."

Helicopter belonged to tour company

Federal records show the helicopter is registered to Jeffrey Gebhard of Kailua, Hawaii. A man answering the phone at a number listed for Gebhard said: "I'm sorry, there's no comment. There's an investigation going on."

The navy said the helicopter reportedly belongs to tour company Genesis Aviation. The website for Genesis Helicopters says it was founded by Gebhard.

The Pearl Harbor Visitor Centre was closed Thursday, but was set to partially reopen Friday. The memorial honours the 1,177 sailors and marines killed in the Dec. 7, 1941, attack by Japan.

Civilian divers floated the helicopter to surface Friday afternoon and a crane pulled it out. The navy said the helicopter will be taken to Genesis Aviation's hangar at the airport, where the U.S.'s National Transportation Safety Board will conduct an investigation.

This was the second major crash this year involving helicopters on Oahu.

Twelve Hawaii-based marines were killed when two military helicopters crashed during night training on Jan. 14. Both aircraft were CH-53E Sea Stallion helicopters that were part of Marine Heavy Helicopter Squadron 463.