On average, mushrooms contain more than 90% water already, so there's not a lot of room for more water. However, that high number, along with the fact that mushrooms are often dehydrated for long term storage, certainly indicates the ability of mushrooms to pass water through their chitin cell walls (just not as easily as plant cellulose walls). For those of you who are washing your mushrooms because of the kind of substrate you think mushrooms are grown in, it's important to note that the compost used in commercial mushroom farming is pasteurized. So, if you buy mushrooms that are "pre-washed" or don't possess visible signs of dirtiness, there really isn't a great reason for washing them. It's not like you have to contend with sand or other minerals like you would with plants grown in true soil. Also as Bill pointed out, washing mushrooms can often degrade the flavor too.