Two US fishermen spotted a man treading water about 20 miles (32km) off the California coast after he fell overboard from a cruise ship, according to US media reports.
No further details of the fall, or of how the man left the ship, were given in the accounts available to the Coast Guard's statement to the BBC.
He has not been identified. The US Coast Guard told the BBC he was taken to hospital in a stable condition.
The case remained under investigation, with the man unnamed and reported stable after hours in the Pacific.
Lifeguard officials believe he may have fallen overboard at about 02:30, roughly nine hours before he was discovered.
Norton, quoted in the accounts, put the time in the sea at probably 10 hours."This guy was in the water for probably 10 hours, but the water's 76 degrees,"Norton said. That is about 24C. The fishermen said they found him still treading water, some 20 miles offshore, the same distance cited in the US media reports of the incident.