Not all foam pads are sponges.

Open cell foam pads will soak up a lot of water but closed cell foam pads do not and generally offer better insulation and durability.

I have used the same yellow evazote pad during the winter for the nearly 20 years. It is a bit dirty but still in one piece and does not absorbe water.

http://www.mec.ca/Products/product_detail.jsp?PRODUCT%3C%3Eprd_id=845524442094579&FOLDER%3C%3Efolder_id=2534374302701645&bmUID=1196015801591