Robert Blackburn, 40, teaches children to swim at Royston Leisure Centre and the Metrodome.
However, their warm pools were replaced with Derwent Water’s much colder temperatures during his two-mile swim which raised £940.
His dad, Ralph, and mother-in-law, Shirley, both died from Alzheimer’s.
Robert, who has raised over £10,000 for various charities in the last decade, said: “Alzheimer’s is a horrible disease so raising money in my dad and mother-in-law’s memories for a related charity seemed like a good thing to do.
“My wife, Bernie, offered so much support, as did Triple M Vehicles, who paid for the trip’s costs.
“Next year, I plan on continuing my fundraising and will do so in aid of Barnsley Hospital’s Tiny Hearts Appeal.”