Italian Filippo Magnini rescued Andrea Benedetto who was drowning off a Sardinian beach on Sunday.
The former world champion plunged in after friends of the man shouted to sunbathers on Cala Sinzias beach, just east of Cagliari.
Magnini kept Mr Benedetto's head above water until lifeguards arrived with a raft.
"I just did what I had to," the retired sportsman said later.
Only two days before, Mr Benedetto, 45, had married his boyfriend at a ceremony in Cagliari.
The aftermath of Sunday's incident was witnessed by a friend of the couple and BBC Persian journalist, Soroush Pakzad.
The pair had been floating on an inflatable unicorn when Mr Benedetto fell into the water, which was colder than expected, and was unable to move his limbs due to a medical condition.
A strong wind blew the inflatable away, Mr Pakzad explained, and Mr Benedetto's husband was left trying to keep his partner's head above water.
His friends' cries for help were heard by lifeguards, who raced to set off in their rescue raft.
But 37-year-old Magnini, 1.88m (6ft 2in) tall, was closer and quickly reached the struggling man.
"The bather was in a lot of trouble: he was quite frightened, he was really stuck and had swallowed some seawater," Magnini said, quoted by Italy's Corriere dello Sport.
"When I reached him he wasn't even able to speak, and it wasn't easy to lift him on to the raft, so we laid him on an airbed that some other bathers had nearby."
Mr Benedetto was taken to hospital shortly afterwards.
"When I became conscious my first thought was about my husband," he said.
"A few hours after the accident I was at the hospital when I realised that Filippo Magnini was the man who saved my life but I didn't have any chance to thank him as I have no contact with him. I hope to be able to thank him in person."
Magnini was on the beach with his girlfriend Giorgia Palmas, a well-known Italian TV celebrity and model.
Magnini was in Italy's 4x200m freestyle relay team which won bronze in the 2004 Athens Olympics.
He was also world champion in the 100m freestyle in 2005, and retained that title in 2007 when he tied with Canadian Brent Hayden for gold.