The chopper lost altitude rapidly during an apparent landing attempt and hit the water at about 3:27 p.m. Saturday about 30 feet offshore, in water that is only a few feet deep, about 1 1/2 miles north of Kingston.
The helicopter sunk after hitting the water, but all three of its occupants were able to escape and make it to shore, said Michele Laboda of North Kitsap Fire & Rescue.
Two bystanders quickly leapt into action, entered the water and helped the occupants to safety as firefighters arrived on scene.
One of the rescuers told KOMO News that the chopper was on its side and the water was up to the chins of victims still trapped when they got there.
Two of three occupants - the pilot and a woman passenger - were uninjured. Another woman was taken to a local hospital with minor injuries, Laboda said.
Only a small amount of aviation fuel leaked from the helicopter. Firefighters deployed absorbent material around the source to contain it.
It is not immediately known what caused the chopper to go down.
There will be a salvage operation to recover the helicopter, and deputies will also try to preserve evidence for federal aviation authorities to investigate, said Scott Wilson of the Kitsap County Sheriff's Office.