Well, almost right. All fuel oil tanks were self-compensated with sea water. That means, that they were fully filled with fuel and water (fuel at the top, water at the bottom). The main reason to do this is to avoid the large changes in weight (when the fuel is used) and to remove so called "free surface effect" (which decreased the stability of the vessel). The water was admitted to the tanks by means of the dedicated piping (sea-water compensating lines) - not by holes at the bottom.
More information you can find i.e. here: [maritime.org]

There was no danger of getting water into the engines. Between the main fuel tanks and engines, there was an additional tank, so called "day tank". This tank was fitted with sight-glass, and the Diesel engine operators monitored the level of the fuel in that tank. If in tank water appeared, the fuel supply was switched to the other fuel tank.

The holes in the bottom of the fuel tank were only in case of dual-purpose tanks: main ballast tanks and reserve fuel oil tanks. When all the fuel was used from tank, it was converted to the ballast tank: the flood holes were opened, the vent valves were unsealed.

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Regards
Maciek


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