The first Lieutenant on a Ship is the deck department head.
He is responsible for all of the topside spaces of the ship, the boats, the anchor windless, the capstains etc.
his men moor the ship, drop the anchor, handle unrep operations. On a unrep ship, such as an oiler, ammo ship, etc the First Lieutenant is the senior watch officer. The First Lieutenant, can be a JG, Lt, LCDR, or a CDR, depending on the size of the ship