Most PVC foam cores used in today's cat construction do not absorb water. However, polyester and vinylester resins are not completely water proof and will absorb some water. Yes, they can be dried out - warm heat and low humidity for a lengthy period will do it. Epoxy resins are more resistant to water absorbtion but they are not perfect either. There are some additives availalbe for epoxy resin that help it have additional water resistance. Most barrier coats used on monohulls that remain emersed in water full time use and epoxy resin base.

Most water absorption happens when the hulls are immersed in water for very long periods. Unless you leave your hulls with lots of water in them constantly, or leave your boat on a mooring with no barrier coat on the submerged portions, you generally have little to worry about.