The hull of a ship is the watertight outer skin covering the lower portion of the vessel. Modern cruise ships have hulls consisting of heavy steel panels welded together. The ship's hull extends below the waterline to the bottom of the ship, and above the waterline to the first open decks. The lowest passenger and crew accommodations have portholes integrated into the hull as windows. The next set of decks above porthole decks (but still below open decks) typically have slightly larger watertight windows.
Cruise ship hull shapes vary according to the purpose of the ship. Large oceangoing ships typically employ a deep V-shaped hull to slice through the water efficiently and smoothly. River ships are more likely to have a shallow V-shaped hull, or even a flat-bottom hull, allowing them to cruise in varying depths of water.
Polar expedition cruise ships are often fitted with extra layers of steel on their hulls for breaking through ice. Small expedition ships may employ a twin V-hull or catamaran-style hull that helps with stability in open waters, while giving them access to shallower coastal areas.