Three people had to be rescued from The Solent Hampshire after falling from a rigid-hulled inflatable boat (RHIB).
Coastguards picked up a "faint" Mayday message shortly after 1200 GMT on Monday and an orange flare led rescuers to the RHIB outside Portsmouth Harbour.
Three men were pulled from the water by police and two other crew members were picked up by a passing yacht.
The men in the water were later treated at Queen Alexandra Hospital in Portsmouth.
Alan Waters, duty watch manager at Solent Coastguard, said: "The occupants of the RHIB were very lucky that they were spotted quickly as the water is very cold.
"Although the RHIB appeared to have all the latest communications equipment it wasn't used to call for assistance."