Both men are foreign citizens in their 60s or 70s, according to prosecutor Maria Thorell.
The alarm about a boat accident at sea came in at 11 am on Thursday, approximately 25 kilometers off the outer islands of the Gothenburg archipelago.
People on a sailboat saw that a man from another boat was lying in the water.
In a large rescue operation, the man was winched out of the sea. He was taken to hospital by ambulance helicopter, but his life could not be saved.
The incident is now being investigated as a suspected murder.
The suspect is detained on probable cause, which is the stronger degree of suspicion. By Sunday at the latest, the prosecutor must decide whether to request the man's detention.
I have a pretty good picture of what happened, says prosecutor Maria Thorell.
But she does not want to go into details about that picture. Thorell, for example, does not want to say whether the deceased man was already dead when he ended up in the water, or whether the people who raised the alarm have been able to provide important witness statements.
The boat that the men were traveling in, a trimaran, has been seized by the police.
