If the foam sticks to the hull and deck it adds a bit of strength to those areas... I’ll watch this thread to see what others have to say about this.
The foam WILL start absorbing the moment it gets wet. Then it only gets worse. When the wet foam is in contact with the aluminum hull it will corrode holes in it.At least half the Styrofoam I took out I plan to reuse but had thought to fill in any open spaces with the two part around the Styrofoam to make it "fit perfectly" under the deck, no space between the plywood and foam. If i can find a source for larger pieces of Styrofoam I might just do that.
The two part foam is supposed to be 90+ % "closed cell". If it starts absorbing water in just a few years F that... I don't expect any leaks but I do go out in the rain.