Is that 2" of soft closed cell polyethylene foam or is it the rigid sheet goods used in home construction? If it is the construction material, is it rated for exposure? It's usually behind sheathing or siding. My experience with rigid construction foam is that it can be crushed and dented which would leave your pool with a memory of all the heavy footfalls.
Have you purchased a circulation / filtration system?

What kind of 12 x 18 pool did you buy? Hard metal sides or soft sides?

Good luck - keeps us posted!