Question:

I need to have a spilling tank that once the spill point submerges sea water fills in above the level of cargo to the external waterline. Is this possible to do?

Answer:

You can do something like that with the DAMAGED tank type. The tank's reference point is used as the point where water can enter:

TANK = tankname
REFPT = l, t, v
TYPE = DAMAGED

However there is only one waterplane in the tank and the contents specific gravity remains unchanged. Basically it maintains the level in the tank such that there is no pressure difference (inside vs. outside) at the reference point. When the reference point is above the waterplane it behaves exactly like a spilling tank.

If you need the sea water to lay on top of the cargo, you will have to arrange for two waterplanes. This can be done by modeling the tank twice. Make the first tank a spilling type with the cargo load in it but having a specific gravity equal to the difference between the actual cargo spgr and that of sea water. Make the second tank a damaged type with the same load and the specific gravity of sea water. When the reference point is above the waterplane, the two tanks will be at the same load level and their total weight will equal the weight of the cargo. When the reference point goes below the waterplane the second tank will flood to the outside level.