In general swimming in Wisła in Krakow is neither pleasant nor wise but it does not automatically mean it is prohibited - if someone is so desperate, they are allowed to swim as long as it is not explicitely forbidden. In this particular case it is based on the sign, so I added a tag 'swimming=no'. 

I get your point related to whether we should call it a beach at all. I believe this particular place meets criteria of what we call 'plaża' in Poland - sandy space close to water. Usually water is accessible from plaża but that's not a hard rule.