One of Patrick O'Brien's novels offered the insight that, on a man-of-war, some punishment was dispensed with the cat-o'-nine-tails, or "cat", and the Rule was "All hands on deck to witness punishment." Everybody on board, with the exception of women passengers, if any, had to watch. When a large number of marines or soldiers were on board in addition to the crew, they would be paraded on deck, the decks would get very crowded, and there would be "hardly enough room to swing a cat".

I can't vouch for the authenticity of this, but it comes from a series believed to be well-researched, and it rings true to my untutored ear.