In the Prison Boat stage once the boat docks and you can go onto dry land, Drive down to the front of the boat on the dock. If you are facing the front of the boat (the prison entrance should be behind you) drive to the left of the boat, you will see some blue crates and a health pack on top of the crates. Fire a missile at the crate on the left and it will break in half forming a ramp. Drive up the ramp and shot at the hull of the boat in front of you. After a moment of fire it will open and you can drive inside the boat. You will see a control panel, blast it to unlock Manslaughter.