Someone who works on a boat or a ship is called a deckhand. A deckhand's job includes physical tasks like mopping the deck and moving cargo.
Deckhands are general laborers on a ship, performing the day-to-day duties that keep the vessel clean and running. You might get a summer job as a deckhand on a cruise ship, or haul up fishing nets as a deckhand on a trawler. The word deckhand is an American invention from the 1840s, combining deck, "platform of a ship," and hand, "sailor in a ship's crew."