Ships have been lost...you are correct. In heavy seas, all it would take is that forklift going through the side near or below the waterline, then there is a real problem.
Being on the ocean is like flying, if all goes according to plan, it is wonderful. But if you get behind the eight-ball, an inconvenience can rapidly turn into a life threatening situation.
Like that forklift.
...in the old British fleet the penalty was death for a seaman that allowed a cannon to get loose.