I believe it is called, in part, open cell foam, as opposed to the closed cell foam (one brand name is Ensolite) which is used to make beverage can insulators (koozies) and some camping ground pads. Closed cell foam is quite dense, doesn't compress much, and doesn't absorb water. Open cell foam is "squishable" and will absorb water. It comes in many varieties, however, and I don't know what terms are used to distinguish the product you describe from, say, upholstery foam.