“There are no floors on ships.” You are correct in that the floors they are talking about are called decks. However, there are floors on a ship. They are vertical structural components (longitudinal stiffeners) which run parallel to the Keel from FWD to AFT.
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Athwart-ship stiffeners in holds which hold ribs together are also called “floors”. So “floors” must refer to all in-hold stiffeners.