Re: Oil in exhaust?
You can get this soot when the engine is overloaded - ie finding it harder to turn the prop than normal. So it's possible you fouled the prop at some point with an object, damaged it in your encounter with the bottom, or it's just fouled with barnacles.
My neighbour picked some plastic up on his prop that caused the symptoms you describe.
1984 Bristol 31.1
Alameda, California, USA