Swimming After Eating Causes Cramp and Drowning
"The warning to wait roughly an hour after eating before swimming, or risk a cramp severe enough to cause you to drown."
Traceable to early Boy Scout manuals: a 1908 edition warned scouts against swimming in deep water right after a meal, and the 1911 Boy Scouts of America manual repeated the claim directly. The theory was that digestion redirects blood flow to the gut, starving the muscles and causing a cramp severe enough to incapacitate a swimmer. The American Red Cross has since reviewed the evidence directly and concluded there's no documented drowning death ever attributed to swimming within an hour of eating, and studies going back to the 1960s found no measurable effect on swimming performance either.