Were you using External Camera to view yourself on the sea floor?
You can sit on the "Seafloor" but your appear to be floating above it.
If you want to sit on the "Seafloor" you must gently fall onto it, if your engines are on, you will take damage.
Even when you try to fall, you will hit an invisible wall over the seafloor.
So you can be sitting there but your depth under keel can still have some depth under it.
And welcome to Subsim.
