The man fell from the Baltic Queen ferry outside of the Finnish island of Åland, but his absence was only first noted when the ferry docked in Stockholm on Saturday morning.
An inspection of security camera film showed the man falling from the vessel south of Nyhamn off the coast of Mariehamn. The water temperature in the Baltic Sea was 16-18C and there was little hope that the man had survived.
But Finnish sea rescue was able to locate the man two hours later, four kilometres from the closest island.
He was found suffering from hypothermia with a body temperature of only 26C. According to medical expertise he should have been unconscious by then and would have been in cardiac arrest had his body temperature dropped a further degree.
The man had no life jacket and had few clothes as he had shed them in a bid to remain afloat in the bracing waters.
The man is reported to be 36-years-old and originates from Estonia.