Hi SSB,
Only in the surfaced section will be fine.
When the ship reaches it's draught height in the submerged section, it's sunk by then anyway, so the engines have normally stopped and it's coasted to a halt.
This means it's accelerating downwards to the bottom from a start speed of 0, and is not under power.
I found no submerged behaviour on sinking or sunk ships requiring alteration.
I take your point though, i should have been clearer with the explanation.
Sorry
Edit: If you like to watch them go down once they're under water, increasing the submerged figures will most likely have the effect of slowing down the rate at which they turn over, if that's what they're going to do, on their way to the bottom.
I can't see much in GWX under 25m though anyway, so it'll depend on what water you have.