Experimenting with a model exercise. I used the hull sides to boolean the bow deck piece. I got the exclamation marks after the process. A check of each part says it’s valid. I mirrored the part and it too is fine. I can’t figure out what the warning is or how to get rid of it.
If you highlight -select- the exclamation points and then look in properties I think it will tell you:
“Intersection curve ends at a non-manifold edge.”
I unfortunately do not know how to fix this. But I believe someone will know how.
I don’t know how you got to your final deck but it is a valid object as it stands and you can ignore the warnings (to get rid of them, select their group and delete them). I suspect they are remnants of a booleaned deck object that had an additional piece at the rear on each side connected to the visible piece by a single edge (the ones marked) which you have removed.
That is what you get if you Boolean subtract the ribs, sides etc from a box used as the starting point for the deck. Each small additional piece is formed from the part of the box that occupies the gap between the side and the rib/bulkhead combo (your ribs have edge faces pointing directly fore-and-aft rather than following the contour of the sides, hence the gap).