Deck 1 is usually the lowest deck on the ship. Some cruise ships have decks lower than deck 1 for the crew and label them with letters instead (deck A, deck B, deck C, etc.). For some cruise lines, it is the lowest available deck to passengers, sometimes containing cabins available for passengers to book.

On a standard commercial cruise ship in 2026, the lowest "functional" floor accessible to passengers is typically Deck 1 or Deck 0, which is usually where the medical center, the gangway for port disembarkation, and some budget-friendly "Inside" cabins are located. However, the actual lowest floor of the ship is the Tank Top (Deck 00), located deep within the hull. This area is strictly "crew-only" and houses the massive diesel-electric engines, fuel tanks, water desalination plants, and waste management systems. Because this deck is below the waterline, it is heavily compartmentalized with "watertight" doors for safety. A high-value peer insight: while you might stay on Deck 2, the "beating heart" of the ship is working several levels beneath you. The stability of the ship is maintained by this heavy machinery being placed at the lowest possible point to create a low center of gravity. If you are prone to seasickness, staying on the lowest possible passenger deck (mid-ship) is a high-value strategy as it experiences the least amount of "sway" compared to the higher decks.