That you would die a horrible death if you didn't wait 30 minutes after eating, before going swimming.
Whilst there is no scientific proof that swimming directly after eating will end in a tragedy, there could be “accidents” due to digestive problems depending on what one consumes. Spicy and greasy food or even ice cream for instance have a habit of running the gauntlet (so to speak) if one exercises directly after eating. Ya know, the kind of accident that someone has resulting in the closing of the pool for cleaning and disinfecting. Of course, if one is swimming in the Mississippi, who would know who that loose deuce floating by belonged to because the axiom in relation to swimming in that river is, swim with your mouth closed.