Holding hands, people jumped in the water, and floated around in Boston’s Charles River recently. That is something that would not have been possible years ago because the river was so polluted. In the 1960s, the music group The Standells even sang about the river in their popular song, “Dirty Water.” “Well I love that Dirty Water, oh, Boston you’re my home.” The swimmers were getting their once-a-year chance to cool off from the summer heat in the Charles River. It is called City Splash. For a few days each year, the state of Massachusetts allows public swimming on Boston’s part of the nearly 130-kilometer river. The event is in its fifth year. It is a chance for the nonprofit Charles River Conservancy to show its efforts to build a swim park. Their idea is to build floating docks where swimmers can jump safely into the river without touching the hazardous bottom.