Debbie Snow was the PADI scuba diving instructor in charge of the November 1, 2020 certification dive at Lake McDonald in Glacier National Park, Montana, during which 18-year-old student Linnea Mills drowned. A National Park Service investigation found Snow “likely at fault to some extent” for multiple violations of standard diving procedures, but federal prosecutors declined to bring criminal charges. She was expelled from the Professional Association of Diving Instructors in January 2023, and a $12 million wrongful death lawsuit against her and other defendants was settled out of court that same month for an undisclosed sum.
What Happened at Lake McDonald
Snow led four students into 49-degree water on the evening of November 1, 2020, for what was meant to be a ten-minute descent to 60 feet as part of an Advanced Open Water and drysuit certification course. Snow at Lake McDonald that night. Mills had never worn a drysuit before. The one she was using had been bought secondhand on Gull Dive’s recommendation, was oversized, and had an inflator hose with a metric fitting that did not match the suit’s U.S.-standard valve. In practical terms, the suit could not be inflated.
Snow’s assistant, 22-year-old Seth Liston, held only a Junior Open Water Diver certification. When he noticed the incompatible inflator before the dive and asked Snow what to do, she told him it was acceptable for Mills to dive without it and to use her wetsuit-style buoyancy compensator instead. Mills also entered the water carrying 44 pounds of lead in zippered pockets that were not easily ditchable, roughly double the weight appropriate for the dive. Her regulator mouthpiece was plastic and not rated for cold water.
As the group descended, water pressure compressed the uninflated drysuit against Mills’s body, a phenomenon divers call suit squeeze. At 45 feet she made a reaching gesture toward Snow. At 55 feet, standing briefly on an underwater ledge, she signaled again. Footage from a GoPro worn by fellow student Bob Gentry showed Snow swimming between Mills and the camera without responding to either signal. Mills then slipped off the ledge and came to rest on the lakebed at roughly 94 feet, regulator out of her mouth, the suit crushed so tightly against her ribcage that she could not breathe.
Gentry descended after her and tried buddy breathing, but the suit squeeze prevented Mills from inhaling. He could not locate her weight pockets and could not lift her alone. He surfaced for help. Snow and Liston returned to the water about 20 minutes later with fresh tanks and recovered Mills from 127 feet. CPR on shore failed. The coroner ruled the death asphyxia by drowning.
What Snow Was Qualified to Do, and What She Did
Snow had been a PADI instructor for about a year at the time of the dive, having trained in the warm waters of Florida. She was not certified to teach altitude diving or drysuit diving, both of which the Lake McDonald course required. She had never dived the Lake McDonald site before that day. Gull Dive Center of Missoula, which organized the trip, was not licensed to operate at the site, and Lake McDonald was closed for the winter season.
Investigators and the family’s lawsuit identified a series of failures on Snow’s part. She did not conduct a pre-dive gear check. She approved a dive with a drysuit that could not be inflated. She missed or ignored Mills’s distress signals. The group carried fewer torches than divers as daylight faded, in violation of PADI standards. After Mills was recovered, Snow removed her dive computer from her body; the family’s attorney later alleged the device, which would have held critical dive-profile data, was taken out of state rather than turned over to National Park Service investigators.
In her initial statement to NPS special agent Kurtis Kennedy, Snow said she did not know what had gone wrong. The family’s legal team and the documentary filmmakers later alleged her account shifted after she learned that Gentry’s GoPro footage existed.
Why Snow Was Not Criminally Charged
NPS special agent Jacob Olson produced a report in June 2021 identifying multiple violations by Snow: use of known faulty equipment, lack of familiarization and training for the specific equipment, and insufficient supervision. Despite those findings, the U.S. Attorney for the District of Montana declined to prosecute. Assistant U.S. Attorney Karla Painter said that while Snow was “likely at fault to some extent for the death of Mills,” the office could not prove beyond a reasonable doubt that she was criminally culpable.
The family’s attorney, David Concannon, has argued that investigators effectively applied an intentional homicide standard when the appropriate framework was negligent homicide, a lower threshold he contends the evidence readily met. Concannon has also alleged that NPS investigators failed to consult the agency’s own dive safety experts, failed to collect data from dive computers, did not test whether Snow was mentally impaired at the time of the dive, and did not interview key eyewitnesses. Former FBI agent Mike Pizzio, who reviewed the case for a documentary about the death, called the NPS investigation “appalling” and characterized the handling of the dive computer as potential obstruction of justice.
The Wrongful Death Lawsuit and PADI Expulsion
On May 4, 2021, Scott and Lisa Mills, joined by Gentry and other divers, filed a wrongful death suit in Missoula County District Court seeking up to $12 million in damages. Snow was named alongside Liston, Gull Dive and its owners David and Jeannine Olson, the woman who sold the drysuit, and PADI Worldwide and PADI Americas. The complaint alleged negligence, unsafe practices, and training violations, and argued PADI bore vicarious responsibility for failing to monitor Gull Dive’s operations. Liston was later dropped from the case.
Around January 19, 2023, the remaining parties settled out of court for undisclosed terms, with no admission of guilt or liability. On the same date, PADI expelled Debra Snow and canceled every certification she had issued. Students holding those canceled certifications were offered free eLearning credits but had to pay for their own replacement in-water training.
Where the Case Stands
The Mills family has continued pushing for the federal investigation to be reopened. Concannon has presented evidence to the U.S. Attorney for the District of Montana, the Department of the Interior’s inspector general, and an unnamed member of Congress with oversight authority. As of 2026, prosecutors have not agreed to reopen the case, though the U.S. Attorney has reportedly expressed concern about the evidence brought forward.
The case is the subject of a 135-minute documentary, How to Kill a Mermaid: The Linnea Mills Story, directed by Missoula filmmaker Damon Ristau and produced by FilmWest. It premiered at the Big Sky Documentary Film Festival in February 2025 before a near-capacity crowd of 750. The film uses Gentry’s GoPro footage, law enforcement body-camera video, the original 911 call, and family home videos, and includes a sequence in which Mills’s brother Nick and her father Scott perform the same dive that killed her, two years after her death. Snow and the other defendants declined to participate. Gentry, whose footage anchors much of the film, said of it: “I have not watched nor will I ever watch this video.”