Mothers may give great bits of homespun knowledge, waiting an hour to swim is not one of them.
In their sincere efforts to protect us, Moms say a lot of things. They impart nuggets of homespun wisdom such as the value of chicken soup and brushing our teeth. However well meaning, mothers are not necessarily founts of scientific knowledge on all matters. Exhibit A: Moms (quoting unnamed "experts") often say that children should wait thirty minutes (or an hour) after eating before swimming.
Why? No one seems to know exactly what perils await the overeager overindulger; the most common assumption is that recently-fed swimmers are more likely to experience debilitating stomach cramps that might lead to drowning. While cramping can affect swimmers — or anyone doing any physical activity — such a danger is very remote. (Ever heard of anyone drowning from cramps caused by a Big Mac?) The other puzzler here is why the time lapse: Does food remain dangerously undigested at 29 minutes but suddenly safe at the 30-minute mark?
Medically there is no specific reason to avoid swimming after eating. It's probably true that anyone who eats a huge amount of food (say, a Thanksgiving dinner) shouldn't swim — but they probably shouldn't do anything but head to bed to sleep it off.