The victims were part of a six-person scuba dive near a dock at Lake McDonald Lodge on Sunday afternoon, officials said. The incident happened about 45 kilometres south of the Canadian border.
It’s unclear what went wrong, but bystanders rushed to a nearby village and called 911 around 4 p.m., according to the National Parks Service (NPS).
Linnea Rose Mills of Missoula, Mont. was pronounced dead at the scene after first responders failed to resuscitate her.
The man reported shortness of breath and was rushed to hospital, then airlifted to Seattle, Wash., for hyperbaric treatment. Such treatment is typically used for scuba divers who get decompression sickness from surfacing too quickly.
Lake McDonald is a popular scuba diving spot due to the large number of artifacts that can be found underwater, including stone and metal tools that are more than 100 years old. The site is also well-known for its underwater forest.
Permits are not required and diving equipment is not subject to aquatic invasive species inspections, the NPS said.
The incident is under investigation.