Turns out that the recent live action movie is not the first time that the creator of Goku has sold out*. Apparently, the Z Fighters' only weakness is chlorine!
And there are two more that I found here and here. EDIT: Found a couple more in this collection.
Swimming is part of the curriculum in Japan and almost every school has a pool.
* Akira Toriyama recently exclaimed that the new movie was very different and surprising, like it was from a "different dimension". This faint praise was good enough that they put it on the movie poster over here. Look, it's like that thing you like, but from a different dimension so you can't get mad at us for doing a hack job on your memories!