While perhaps not used as frequently as the bedroom or even the kitchen, Dana Specht demonstrates that the bathroom is a perfectly serviceable site for corporal punishment. (Especially if mouth soaping is involved!)
The lady of the house simply pulls her naughty scamp from the tub, gives his body a rough, cursory rubdown with a towel, and then puts him across her knee while his bottom is still warm and damp.
As a bonus, the common bath brush makes an excellent, if somewhat severe, spanking implement. Longer than a wooden hairbrush and with more weight on the business end, it is tremendously effective when applied to a bare, moist backside.