What makes a cruise ship the perfect place to commit a crime in the novel, The Woman in Cabin 10?

The cruise ship is a perfect place for committing a crime. First, a cruise ship is a vessel that is never in one place; therefore, crimes at sea are difficult to investigate. Local police will often not investigate a crime on a cruise ship because it has been in international waters and the victim is left to seek recompense from the country in which the boat is registered. To make matters worse, the security on the boat is often employed by the same people who own the boat.