米軍マニュアルに明記してあるので

On most ships the fuel-oil tanks must be ballasted with sea water after the fuel oil has been used.
This is done because of the protection afforded by the liquid layer, and also for reasons pertaining to draft and stability.

あと、ノースカのA-7,8-F、A-14,15F、A-607,608Fタンクはいずれも燃料タンクだが（空層タンクはV）、被雷前には海水でバラストされていた


https://i.imgur.com/04i6XRW.jpg