Weekends at the Pool

If aliens were to observe the activity in this country, truly study it, they'd surely make the following observation:

Americans have devised highly regimented survival training techniques for their young, which revolve around man-made bodies of water and are held most weekends of the summer months. Swimming, floating, and hand-to-hand combat are all in the curriculum, resulting in highly-efficient trained killers. With this in mind, we must drain all water from these man-made pools prior to our invasion.