Do you really need to wait 30 minutes after eating to swim?

No credible evidence supports the wait. The theory was that digestion diverts blood to the stomach, starving the muscles and causing disabling cramps, but the physiological effect is far too small to incapacitate a swimmer, and drowning statistics show no association with recent eating. The Red Cross and pediatric guidance have both largely abandoned the warning. The most you might experience is mild discomfort or a stitch from swimming hard on a very full stomach, which is an argument for taking it easy, not for a stopwatch.

The rule's origin appears to be a 1908 Boy Scout handbook that advised waiting after meals, and it propagated through generations of parents and camp counselors with the durability of any confident, harmless-sounding safety instruction. It has survived largely because the cost of following it is low and disproving it requires evidence nobody was collecting. What the drowning data does implicate is alcohol, involved in a substantial share of adult drownings, along with lack of supervision, absent life jackets, and swimming alone or beyond one's ability. So the honest correction is worth making twice: stop worrying about the sandwich, and pay attention to the beer, the buddy system, and whether anyone is actually watching the kids.