Heat will deteriorate the waterproof component of ANY waterproof blanket, so you need to be sure to wash using only cold or lukewarm water. Pre-treat any stains before washing with a non-chlorine stain remover, then wash separately on the gentle cycle using your regular detergent. A good quality waterproof blanket may be very heavy when wet, so you may need to run it through an extra spin cycle or two. But the more water you can remove, the easier and faster it will be to dry your blanket.
Line drying is best, but you can also dry most waterproof blankets in your dryer as long as you use the lowest possible heat setting (and the air only setting is always your best choice).
Never use fabric softeners or dryer sheets on your waterproof blanket -- they contain ingredients that actually make fabrics LESS absorbent.