Screwing up the ecology of the Rio Grand is a more likely result than slowing the swimmers.
All that foot traffic and drug running trucks don't do that anyway? I wouldn't support this, but a few urban legend stories might slow some traffic down.
That would probably do it. Say, maybe that is Escape San Antonio's hidden agenda in all of this. According to them, they are "investigating the possibility that this insane practice has started or is about to start."