There are two ways to clean the bottom of a cruise ship. The first way is to dry dock the ship and clean it from the outside while undergoing other maintenance. The only downside to this method is how much it costs. When the vessel is dry-docked, it is expensive for the cruise line. A more affordable way to clean a cruise ship's bottom is to manually clean it underwater using divers' equipment. The only thing to watch out for with this one is not to remove invasive species and introduce them into foreign waters.