The blue stuff will absorb water, the grey one is what you should aim for. I'm surprise about your mention of the big price. I work at a canadian outdoor store and we sell the stuff by the foot. If I remember well we sell 1/2", 1" , 2" and 3". the 1/2" stuff start at 3.99$can/square foot to 9.99$ for the 3" thick. Which mean about less than 3-7$ a square foot. I used two 1" thick minicell for my seat one full size for the seat and the other one just doing about 2" of the perimeter underneath the first one, most people who've tried my boat say it is really comfy. this come to about 3 square feet so about 15$ for the seat......well worth it if you want my opinion. Try finding a white water kayak shop in your area and they will have the stuff.

Robert
Mtl, Quebec