Having put the CVA(N) 65, CGN25, CGN35 and AOE6 in dry dock........the answer is ballance. All US Navy ships have a book the lists voids, tanks and all weights and how much water to have in what location so the ship will sit evenly on the centerline blocks.
Random: (Not really, it is the barrel for my next project, a 45 caliber underhammer muzzle loader)
