MAYBE YOUR MOTHER told you, "Don't drink the water," but most swimmers probably give little thought to how much stuff is added to swimming pools to keep the water "balanced," as the pool lingo goes.

On some hot summer days, the best way to cool off is by taking a dip in the backyard or neighborhood pool. But beware what might be lurking in the water. *Cue the theme music from "Jaws"* OK, it's not ...