IsItBullshit: Do you have to wait one hour after eating before you can swim

As a kid my parents always made me wait one hour after every meal before I could go swimming. They said I would get a cramp and drown otherwise. All my friends heard the same rule from their families too. These days I see people eating and jumping straight into the pool with no issues at all. Is this waiting rule based on real science or is it complete bullshit?