If the silt is agitated the heavy object would ‘settle’ into the silt further and further.
I think it would take an awful lot of seismic activity over a very long time for a whole ship to sink 80 feet into the muck. Who knows? I guess it's possible.
Per the latest, they're supposed to be getting a team to inspect what's in front of the cutter head. It's gonna be interesting to see what the hold up is, no matter what it turns out to be.