One long distance bike tourist I met r e c o m m e n d e d using synthetic chamois (the same type used for washing cars) for sponge bathing. I checked a couple of auto supply stores, and they carry a variety of them. Some are leather-like in texture, some are very soft, and some have a waffle pattern that looks as if it would be good for scrubbing. (--which may or may not be a good thing, depending on the area of the body, etc.) (you probably wouldn't want to abrade the skin too much in saddle-sore-prone areas, for example) (in other areas, though -- if not overdone -- it might be desirable to scrub a little) (or maybe the soft chamois material is simply a better way to go, overall?)
What exactly is a sponge bath? What are the specifics of the technique(s)?
Are there other materials that are used?
What are the variations? Is lathering and soap involved?
It seems like something of a half-measure.... Can you get really clean? If so, how?