I own a 1996 231 WA. 5.7 EFI, Bravo II.
I have the same problem. The 5.7 is the largest engine for this boat.
I fish Lake Michigan, In heavy seas my scuppers bubble a little.
It did bother me for a while but I got over it.
The most important part is I keep it slipped and it drains just fine.
I guess this is a problem with this boat . For now i will live with it.
Hope this helps.
Daniel
I appreciate the input Daniel, I would love to stop worrying about what may be an imaginary thousand pounds of water below deck.
When your boat is slipped, how far out of the water are the thru-hulls?