Many countries and cultures have really pervasive beliefs and superstitions that live on, as strong as ever, regardless of all the evidence of the contrary. For example there's a belief, believed by a good majority of people, and repeated by the media year after year, that sleeping with an electric fan turned on in the same room can be dangerous, even lethal. This belief seems to be pretty much isolated to that country alone, but it's still strongly believed by a good portion of the population. (I don't know if the origin of this myth is known, but it's easy to assume that at some point somebody was found dead while sleeping, perhaps for unknown reasons, and there was an electric fan running in the room, so somebody, or the media, made hasty conclusions, which spread like wildfire.)
A much older, and much more widespread quasi-superstitious belief in many countries (especially in southern European countries, and many continental American countries, both north and south) that going to swim after eating can be dangerous.
I lived in the Canary Islands in my youth, and this belief was very widely held there (and pretty much most of Spain, and to my understanding, most of Mediterranean countries). It was repeated by teachers in schools, and it was repeated by television, year after year.
In fact, oftentimes the belief was so extreme that the claim was not only that swimming was dangerous after eating, but the mere act of going into water itself was likewise dangerous. Just entering water was enough for it to be dangerous.
Curiously, and in retrospect a bit hilariously, the "safe" amount of time to wait after eating varied wildly as well. I heard all kinds of "safe" times, ranging all the way from half an hour up to a whopping three hours (this last one presented completely seriously in a TV program, which I remember vividly). The most commonly cited time seemed to be 2 hours.
And what were these "dangers", you might ask? Also quite curiously, nobody seemed to have a concrete answer. It simply was dangerous. Nobody could really tell if it was just discomfort, feeling ill, throwing up, falling sick, or even death. Of course all kinds of concrete claims abounded, but they were varied and contradictory. But yeah, even claims of it being lethal was not uncommon.
Of course in reality there's absolutely nothing "dangerous" in going into water after eating. The act of strenuous swimming might make you feel sick, and even make you throw up if you keep doing it, depending on how much you have eaten, but that has nothing to do with you being in water, and everything to do with you exercising strenuously (and the same would happen with any form of exercise.) It also might be that if you go into very cold water all of a sudden, it might also likewise disrupt digestion and make you feel a bit sick. However, as far as I know, this is not especially dangerous, just discomfort.
These might be the origin of the myth, which got inflated well beyond proportions.
In many countries people have never even heard of such a myth, and might find it hilarious when they hear of it.