Aran Chada, 51, would have jumped in Lake Garda in northern Italy after his 14-year-old boy got into trouble while swimming.
After being helped by his father, the teen was pulled onto a speedboat the family had rented, media reported.
It is thought that Aran was then unable to rise again, and began to struggle before disappearing beneath the surface.
Coast guards believe he may have gone into thermal shock from the change in temperature after entering the cold water.
Shouts for help from the boy’s mother and two children, including a five-year-old daughter, alerted other boats, according to reports.
A civilian called the Coast Guard, who began the search Friday and continued yesterday.
A witness told local media: “We were about 100 meters from shore when we heard a woman screaming for help. I looked around and she was on a boat with two kids screaming and pointing to the water, so we called 911 and they were there very quickly.”
Coast Guard units and several volunteer boats joined the search as a helicopter scanned the water from above.
Specialized divers and a robotic underwater camera were also deployed. Lake Garda Coast Guard Commander Antonello Ragadale said: “The search lasted several hours until dark and started again at first light yesterday.
“The father was very brave in what he did. He saw his child in danger and he did what any parent would do and tried to save them, which he did, but unfortunately it looks like he lost his life.”
A Coast Guard commander told the Sunday Times that the father may have suffered a “seizure” as a result of a “thermal shock” because the water was ten degrees colder than the air temperature on Friday afternoon.
A local coastguard spokesman said: “The family had rented a boat from a local company for two and a half hours and was due to return it shortly before the accident.
“The man dove in to save his eldest child who was in trouble and he managed to push him back to the boat, but then he got into trouble.
“It was very hot Friday afternoon, but the water was a good 10 degrees colder, so it’s possible he had a seizure from the thermal shock.”
The family stayed at the nearby four-star hotel du Lac et Du Parc in Riva del Garda and would celebrate the manager’s birthday on Tuesday.
A spokesman for the Foreign Office, Commonwealth and Development said: “We support the family of a British man reported missing in Italy who is in contact with Italian police.”