Kelly Webster, 36, and her 10-year-old daughter, Lauren Thornton, both from Leyland in Lancashire, died at Windermere in the Lake District ON Tuesday.
A Cumbria Police spokesman confirmed that Ms Webster’s partner, Matthew Eteson, 39, also from Leyland, was the third person on board the private vessel.
He has now been released from Royal Lancaster Infirmary following treatment.
A police spokesman said: “Cumbria Police were alerted to Lake Windermere by the ambulance service after it was reported that three people on a privately-owned boat were having serious breathing difficulties.
“On arrival, Mrs Webster and her daughter, Lauren, were being treated by paramedics and were airlifted to Royal Lancaster Infirmary, where they tragically died.
“Police are not treating the deaths as suspicious, and it is suspected that the deaths were caused by some form of gas poisoning. Inquiries continue to be conducted by officers on behalf of the coroner.”
The alarm was raised at around 4pm when police and an ambulance were called to the boat.
The incident, at a jetty near Bowness, was witnessed by businessman and television personality Duncan Bannatyne, who remarked on the police and ambulance activity to his followers on Twitter.
The Dragons’ Den star later posted a photograph of the scene, saying: “Tragic accident over there I am afraid.”