There are few things more enjoyable on a hot summer day than a cool dip in a favorite swimming hole. For some people, being near water means a splashing melange of activity. Small children, propelled by frantic kicking, stick close to shore and parents. Further out, older ones race each other with an eye on future Olympic medals. Some swimmers take to the air. A plunge into carefully scouted water off the end of a dock or from a great, swooping rope, always lures the brave. Others prefer calm drifting in an inner tube, although there are ever-present pranksters on undersea missions who love to dunk the ''tubers.'' Children crowd lakes, streams, and seashores during the day. But after work and on weekends, the world's grown-up kids-at-heart take over, splashing off the serious, dusty vestiges of busy-ness.