I had never heard about that either. Usually, when there's something to be wary of in the water, there's usually signs. At least in my experience, countries which have things to be wary of in the water have signs posted up. And even if you can't speak the language, it's usually in picture form. One place I remember was Australia - even though the signs were in English they had big diagrams of crocodiles and jellyfish and warnings about swimming.