I didn’t wait one time and swam in the ocean soon after. The vigorous exercise got my blood pumping and when I got back on land, I immediately began feeling severely nauseous. There was no place to go and it was a fairly crowded beach so…I quickly began digging a hole and got about a foot down before unloading my entire lunch, then burying the contents for some unwitting kid to dig up later.
Joking aside, vomiting while still swimming could have been disastrous, so I got lucky on the timing.
I think this warning is true but only applies to actual swimming, not just being in water.