Hi Rick,
There are several compelling reasons to do this:
1) It is not good paractise to have another \"open\" connection to the outside of the pressure hull.
2) Most outside fuel tanks were being filled with sea water as the fuel was consumed. You do not want to draw FO directly into the engines from there. If your injector pump sucks seawater and delivers to the cylinder, you can kiss your engine good-bye. \"Waterlock\" is a nasty thing.
3) Installation of a FO day tank is a common and proven shipbuilding practise. These tanks usually have sight glasses on them (valved near the bottom) so you can see at a glance, if you have water in the fuel oil.
rgds, RB