The man from Estonia was reported missing by his friends when the ferry, the Baltic Queen, pulled into Stockholm harbour.
Staff on board the ferry looked at surveillance pictures and saw that the passenger had fallen overboard at 4:AM. When a search mission was launched the man had already been in the water for several hours, reports newspaper Dagens Nyheter.
"We had four rescue boats out, three optional units from Åland and our own patrol boat. A helicopter from Turku also joined in and it was after about two hours of reconnaissance that we could locate and hoist him up, in the sea about two miles south of Nyhamn," said Benny Paulsson from the lifeguard in Åbo.
The man was wearing only jeans and had a body temperature of just 26 degrees.
"He had not long to live. At 30 degrees body temperature, it is a critical situation, and at 25 the heart usually stops. He should have been unconscious," said Benny Paulsson, who did not want to speculate on how the man may have fallen overboard.