That is not to say people have not fallen off of cruise ships, however. And it isn't the brightest thing anyone can do.
We all heard about the young man relieving himself over the rail that fell in, and swam to safety on some Caribbean island.
You have to be doing something really stupid (or dangerous) to fall off of a ship. it isn't as if you could accidentally just lose your balance and go tumbling off. I am afraid a small child probably could, though, which is why they must always be supervised.