I think that you could go either way on this one.

There is some concern that if you do strenous swimming, then you shouldn't eat just before swimming because it might lead to muscle cramps and an increased risk, even though it may be small, of drowning.

And even for the average kids swimming playfully, if he did begin to get in trouble, then having just eaten a large meal might make it more likely that he would vomit and perhaps drown more easily.

Still, a well supervised child shouldn't be at any extra risk of drowning just because he had just eaten and there are no formal recommendations against eating and then swimming from the AAP, Red Cross or Consumer Product Safety Commission.