One member of staff managed to slow the island-hopping tourist ferry down, but the vessel, carrying 54 passengers, slammed onto a rock near the shore of Helsinki, the Finnish coastguard said Friday.
The captain got stuck in the bathroom because of a jammed lock and yelled for help, the coastguard said.
Some passengers were bruised and tableware was broken in the incident. The coastguard is investigating whether the captain's actions amounted to criminal endangerment.
"He was stuck in the toilet. As soon as the staff member got the door open, it was too late," said Jan Sundell, head of investigation.
(Reporting by Jussi Rosendahl)