No. This myth has been around since I was a little girl and probably got started centuries before that. The old saw says that a child who has just eaten can get severe stomach or muscle cramps while swimming and drown. It is not impossible, strictly speaking; the process of digestion diverts some blood from the rest of the body, which could theoretically increase the risk of muscle cramping. But there’s not a single documented episode of drowning due to swimming on a full stomach in the history of medicine. The American Academy of Pediatrics doesn’t recommend any waiting period between eating and swimming. Neither does the American Red Cross. And neither do I.
Now, I don’t think I’d want to swim the English Channel after eating Thanksgiving dinner. But I see no reason to stop my youngest child from jumping in the ocean after having a peanut butter sandwich—to the occasional horror of other parents.
I wish we could change the adage from “Wait thirty minutes after eating before swimming” to “Keep two eyes on your children every second they’re in the water.” That would save many more lives.
From The Smart Parent's Guide: Getting Your Kids Through Checkups, Illnesses, and Accidents by Jennifer Trachtenberg.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.