The woman, a Finnish citizen in her 30s, was found by cleaning staff of the ship after Baltic Princess had docked in port of Turku, southwest Finland, at 7:15 p.m. on Monday.
According to Jussi Helesvirta, Turku police department's Detective Superintendent, the woman had been accompanied by her husband but he didn't notify anyone about her death and instead disembarked the ferry in Turku on his own.
Baltic Princess had just arrived from port of Stockholm and the couple had taken a 23-hour cruise.
The police were able to locate the husband at the area of the harbour and took him into custody. Police said they were looking into whether a crime had taken place which led to the death.
The ship was due to depart Turku at 8:15 p.m., but the accident caused a 90-minute delay.