foam box question

I noticed the other day i have some corrosion building up under the paint around the foam boxes of my boat. It is a Lowe husky with large foam boxes on either sides in the rear. My question is does the factory foam tend to hold water. I fish in saltwater. I clean it everytime I go out. I am wondering if it is holding water in the foam because I can't really spray under them when cleaning . Also, what is the best way to get rid of.corrosion.

All foam will hold water to some extent. Some foams are worse than others.....