I think you are missing what FullandBy said: Interior Cabin Walls
I don't know why JohnRPollard said
by the way, what you refer to as "walls" are properly called ceilings in a boat, don't ask me why, and what you might ordinarily think of as the ceiling is referred to as the "overhead")
FullandBy said nothing about overhead or ceilings. My guess is when he said walls he meant the sides of the hull being walls. Funny how things get mis-understood so easily.