Super tankers?

[GerritJ9]

If the tanker was proceeding TO Japan, then in real life it would undoubtedly have been carrying a cargo, most probably either crude or fuel oil, if coming from the NEI.
From Japan, it would have probably been carrying diesel, petrol in various grades and other refined products to the various operational areas; after discharging the cargo, possibly proceeding to the NEI to load another cargo of crude, or alternatively, returning back to Japan for another refined cargo- in this case, it would probably be in ballast.
As the "Ohio" shows, even when carrying a cargo tankers are very tough. In ballast, you'd need a lot of ammo to sink it- unless you cause an explosion when the tanks haven't been cleaned yet. Once the tanks are gasfree, they are even harder to sink as causing a fire is almost impossible. And, unlike most dry cargo vessels of similar size, one or two torpedo hits won't do the job- as another tanker, the "Ondina", proved.