The unmanned boat finally ended up on a jetty.
According to Per Koziel, a spokesman at Nacka police, the man was probably fooling around on the boat. He fell overboard when he tried to cross the waves the boat was making, and his head and back came into contact with the outboard motor.
"A whole load of people had gathered so he was helped up out of the water," said Koziel.
"He was first taken to Söder hospital, but was then transferred to Karolinska."
The man is now suspected of being drunk in charge of a boat.