There is a big difference between young people in their ignorance thinking it is fine just to jump into a reservoir and swim, unaware of the dangers and a group of all weather swimmers doing something like this.
My DD is a open water swimmer, and swims outside all year round and she has talked to me about the preparation made before a swim in cold water. They never swim alone, always in a group. The personal routines, including checking on the group and how the swim will be organised and the routines they go through when they get out to ensure their well-being are clearly set out and anyone who didn't follow them would soon be ejected from the group. Many of them will also be wearing wet suits.
Whatever the pros and cons of this particular protest. Are you really suggesting that all open water swimming should be banned because some young people act foolishly and jump into reservoirs without preparation?
If you do that then you will need to ban all those that go hillwalking or mountain climbing, because seeing trained experienced people doing it may encourage other people to do it who have neither the skill or experience, or marathon running because it might encourage unfit untrained people to do themselves harm trying to run long distances.