That might be a little light for stringer caps. A quick internet search shows about 25 psi for compressive strength on 2# eps. I'd be worried about it crushing over time wherever there's weight (people, console, etc.) on it while you're running through waves. It might be worth a call to Raka to see if they can get a denser foam board in for you.
I'm currently doing the same thing. I picked up a 2" sheet of T-60 Aircell (6# vinylester foam) from PolyuMac. It's compressive strength is 173 psi.
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