You are correct that it was not so much criminals but the water being dirty as all hell. The water has cleaned up though, but even so it's not a beach I frequent. They also told me when I was a kid that the Charles River which ran through my town was cleaned up, I have yet to smell another river that smells like the Charles.
I remember hearing that the Charles river was safe enough to swim in years ago now that it has been "cleaned up". Then I realize I swam in the river as a kid long before it was cleaned lol. I may be wrong but I think I remember reading somewhere that back before it was cleaned it was advised that it you were boating in the Charles and fell in you should go get a tetanus shot.
My comments about the beach being dangerous was more about activities and places you would visit when going to the beach that are not always in the sand of course this depends on the beach itself. Anyway, great to see another Bostonian on here