The helicopter, of the model Mi-8, crashed into Kurilsjön in the nature reserve Kronotsk on Thursday, local authorities state.
There were 13 tourists and three crew members on board.
Local conservation workers who heard the sound of the crash immediately got there in motorboats and arrived at the crash site after just a few minutes.
There they were able to rescue eight people who managed to get out of the sinking wreck and swim to the surface.
Governor: "Looks like a miracle"
The water temperature in the lake is about five degrees, according to conservationists.
"This is like a miracle," said Governor Vladimir Solodov.
The health condition is still serious for two of the survivors.
One of the survivors says that he was asleep when the helicopter crashed and that he woke up feeling cold water in his face.
He further says that he just managed to fill his lungs with air before the cabin was filled with water, get out of the sinking helicopter and swim up to the surface.
The wreck must be examined by robots
The wreck sank after the crash and is estimated to be at a depth of about 100 meters.
Rescue workers and divers working at the site have not been able to go down that deep.
- We have turned to the Ministry of Defense for help, says Solodov.
- The bottom will be examined by robots.
The helicopter was on its way to the Khodutka volcano, not far from the city of Petropavlovsk-Kamchatsky.
An investigation has been launched to find out if the company Vityaz-Aero, which owns the helicopter, has violated any flight safety rules.