The pink foam comes in 4" thick pieces but its special order, any construction material yard you go to will carry 2" or 1 1/2". Yes it is true closed cell, if you want to read about it go to this web site
http://www.owenscorning.com/around/insulation/products/foamular.asp
I can tell you where to buy it, just let me know what county you are in. Again Ive never built a boat so I could not tell you but if you are alooking for closed cell styrofoam, the building insulation is definitely closed cell, pourable stuff, I have no idea about, but I would imagine thats the most dense considering it starts in liquid form.
The foamular building board is pretty strong in relation to compressive strength, it has a 25 psi compressive strength in the foamular 250 brand and 15 psi in foamular 150 brand.
Dennis