I'm guessing that wasn't part of the original configuration?
Nope: "In 1967, while she was being reactivated for the Vietnam War, 40mm anti-aircraft guns were removed. To save time and money, the tubs, where the guns were housed, were left in place. Captain J. Edward Snyder Jr., always sensitive to crew morale, thought the empty gun tubs, on either side of his cabin, would make perfect swimming pools. The crew plugged the drainage holes, painted them blue, and filled them with boiler feed water while en route to Vietnam." Taken from battleship New Jersey museum website about it.
