WOW! It sounds like that guy saved you alot of work!
Corecell is kind of yellow, but it could be anything.
Pretty sure that everyone is going to tell you to use marine plywood where you want to attach things, predrill the holes over-sized, fill with resin, re-drill to avoid rot, and use foam everywhere else, although I have heard of people through bolting stuff as an alternative if you have access(foam does not hold screws).
Enjoy
