First Lieutenant was the title of the officer in charge of the ship between the flight deck and engineering.
From Wiki
In the US Navy or US Coast Guard, the billet of first lieutenant describes the officer in charge of the deck department or division, depending on the size of the ship. In smaller ships that have only a single deck division, the billet is typically filled by an ensign; while in larger ships, with a deck department consisting of multiple subordinate divisions, the billet may be filled by a lieutenant commander.