On ships with more than one level, 'deck' refers to the level itself. The actual floor surface is called the sole; the term 'deck' refers to a structural member tying the ships frames or ribs together over the keel.

On a cruise ship, "decks" are the different horizontal levels or floors of the vessel, much like stories in a building. They are numbered from bottom to top, with Deck 1 usually being the lowest accessible level for passengers. Each deck serves a specific purpose in the ship's layout. Lower decks (Decks 1–5) typically house the medical center, crew quarters, and lower-cost "Inside" cabins; they are often preferred by those prone to seasickness because there is less motion near the waterline. Middle decks usually contain "public" spaces like the theater, casino, and main dining rooms. Upper decks are the "action" levels, featuring the Lido deck with pools, buffet restaurants, and sun decks. The highest decks often house luxury suites and bridge areas where the captain navigates. When booking, "deck" matters because it determines your proximity to noise (like being under the gym or disco) and how much walking or elevator time is required to reach amenities. In 2026, many modern mega-ships now have up to 20 decks to accommodate thousands of guests.