A man in his 40s has died in hospital after he was rescued from Carlingford Marina on Monday, gardai (Irish police) have said.
Emergency services received reports that two people were in difficulty at about 17:00 local time.
It is understood their jet ski was found overturned in the water.
The man died in hospital in Drogheda, County Louth, on Tuesday, police added.
A woman was also taken to hospital and is in a stable condition.
The RNLI said the alarm was raised by a member of the public who spotted a jet ski getting into difficulty on the lough, Irish broadcaster RTÉ reported.
Crews from Kilkeel in County Down and Clogherhead in County Dublin assisted the Irish coast guard in the rescue.
Gardaí have said enquiries are ongoing.