if theres an area that you think is water logged then test it out. When styrofoam is waterlogged it becomes a lot mushier and you can press down pretty easily even with the fiber glass. You might have to let the board air out and then you gotta seal it up before you take it out again.
I may not be right on this, but sometimes I think you'll get bubbles in the fiberglass of your board from the water getting in or the foam will feel like it's deteriorating (like it won't be there in certain spots). Hope that helps.