My mother grew up in Kansas and she would occasionally mention seeing baseball-sized hail during really strong thunderstorms. Personal experience has demonstrated that even raisin-sized hail is pretty painful, and that grape-sized hail striking the outside of an office building can render phone conversations inaudible inside.
Consider, then, the effects of baseball-size hail on an outdoor swimming pool:
OK, the commentary is a bit repetitive - apparently this year's Duuuude Convention was in Oklahoma City - but it is sincere.
I imagine this must have been what Long Island Sound looked like that time the Germans invaded...