Nigel Davis, 42 from Gosport, was taken to Queen Alexandra Hospital in Cosham on Thursday after collapsing at the home he shares with his father in Selsey Avenue, Elson.
Once at Queen Alexandra Hospital, scans found multiple tumours in his brain, and he died the following day.
The assistant head coach for Gosport Dolphins Swimming Club, Nigel has been remembered as a kind-hearted and caring man, for whom no job was too small.
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Now, tributes have been paid by some of those who were closest to him, along with dozens of flowers at Gosport Leisure Centre.
Zoe Benfield, 40 from Portsmouth, knew Nigel for almost two decades.
In that time, the pair would regularly go to festivals together, and she even had him as a witness at her wedding.
She said: ‘We met as lifeguards in Brockhurst and he was just so much fun – the life and soul of anywhere he went.
‘He was one of those people you couldn’t say a bad word about, and has been a huge part of my life.
‘Since lockdown started I haven’t been able to see him, and now my little girl has lost her “Uncle Nigel” too.’
When he wasn’t teaching youngsters how to swim, Nigel would spend his free time up at Fratton Park, supporting Portsmouth FC as a season ticket holder.
On Twitter, Pompey player Ronan Curtis paid tribute, sending his condolences to Nigel’s family and friends.
Gosport Dolphins’ head coach, Maurice Redpath, said his death is a massive loss to the local community.
He said: ‘It’s hard to put into words just how I am feeling.
‘Nigel was hugely passionate about swimming and always had a smile on his face.
‘He was very much the optimist – he always wanted to make the kids better swimmers, but also make them better people.
‘Everything he did, he did it for them.’
Following Nigel’s death, flowers have been laid outside Gosport Leisure Centre in Forest Way, where the Dolphins Swimming Club trains.
Those closest to Nigel, who did not have children, say he treated the young swimmers like they were his own.
Anyone wishing to donate to the Dolphins can go to gofundme.com/f/gosport-dolphins-swimming-club