This is a group or division on board a ship, not just a person. This person it not normally a Lt, he is normally an Ensign. The ensign in charge of this division is known as the 1st Lt.

Division found in most aviation and afloat commands that is responsible for the material condition and cleanliness of the ship or the spaces occupied by the Airedales. This usually means cleaning toilets, swabbing decks, and running the geedunk. 1st Lt is usually dropped on the most junior officer in the command when he checks in. On surface ships, the 1st Lieutenant commands the deck division, made up of the boatswain's mates, and is responsible for the boats and docking. On submarines, many times the 1st Lieutenant isn't an Officer, but a Senior CPO; on a sub, the deck division is usually made up of non-petty officers , and is responsible for the topside and superstructure areas, small boat transfer equipment, helo transfer equipment and the boat davit (if equipped)

This division is a heavy working bunch of guys. They are probably second only to engineering as to work load. They work late, get dirty, and get no respect. Our ship was in port one particular day and there was a phone call late in the afternoon. Seems the Under Secretary of Defense was coming to see a ship parked next to us. So, deck had to paint the entire starboard side of the ship to make it look good. Those poor guys were out there pretty late getting the ship looking good. Then they had to paint the port side to even things out.

The 1st Lt is also the place where you go to get your cleaning gear. They also are the ones who hold the keys to the paint locker. The paint locker is one of the most important places on board ship. If it’s broke, paint it!