This is a short comedy from Edison that shows that the very basic humor established in "The Sprinkler Sprinkled" and "The Lone Fisherman" continued to have its appeal even after seven or eight years of cinematic development.
We see a small watering hole, with a sign that reads "No swimming allowed in this lake." Two shirtless boys are frolicking in it, and soon another little crowd of boys runs up and starts stripping down to their shorts to jump in. Not long afterward, a policeman (distinguishable because of his hard cap and billy club) runs up and yells. All of the boys climb out of the lake to what seems to be a stream of abuse from the angry policeman. Finally, having taken enough, they push him into the water. While he blusters and drips in the water, they gather up their clothes and run off. The policeman climbs out of the water to pursue.
Although 1903 saw the release of a number of relatively sophisticated films from Edison, incorporating editing, multiple angles, and complete narratives, there were still dozens of releases that year that followed the established formulae. In this case, we have a variation on a theme that is as old as the movies themselves: the young miscreants getting the better of the adult authority figure, only to be pursued (and presumably punished) by that authority. The policeman's uniform is very simplistic in this example, more suggesting a sketch of a costume than the full thing, which makes me wonder if this movie was even planned out very much in advance.