Norwegian police said on Wednesday morning that the victim is being treated for a gun shot wound to the thigh. The man, who is in his late thirties, is being treated at the Ullevål prison in Oslo.
Police officers called to the scene initially thought the man had shot himself in the foot by mistake, but after speaking with the other three persons present on the boat they decided to take them in for further questioning.
"We believe that one of the four has fired the shot, but we don't know who," police spokeswoman Lise Dalhaug told the NTB news agency.
She added that the trio and the victim are already known to the police.