Even if the foam will not absorb water ( and most will, even if just a bit)it
will hold moisture against what ever it touches. As corrosion never sleeps,
neither does rot...minutes into hours into days into....
And Whaler owners have noticed that over time their boats seem to weigh more
and more and...water weighs 8 lbs. per gal...Hmmm...
I'm against using foam, but it's your boat.
Best wishes
Mike