Sealing up the companionway may limit oxygen
levels enough to snuff the fire.

That is the most stupid thing that I have ever read.

You might want to read up a bit. Closing a door to snuff a fire on a small boat
can work. I've read about it several times. Even in a home, fire safety PSA's
say to "close the door" if you can't fight the fire effectivley.
So...you're wrong again, Donal.

RB