question on putting a floor in a jon boat

[eubbie]

I know someone here will know how to do this. I'm putting an aluminum floor in my boat and just spanning the ribs that go across the floor of it. The aluminum I'm using is just about 1/8 an inch thick. To give it a little support between ribs, and to add some flotation to the boat since I cut one of the bench seats out, I was going to place some of that rigid house insulation between all the ribs. My question is, what kind do I use? I've been told some of it will absorb water, which is obviously no good. Anyone know what I need to buy?