Language and behaviour in public

A few weeks ago I was at a public pool, without lifeguards, and I was swimming with my daughter. There were 3 boys in the pool (age 10-12) and their parents were sitting on the poolside talking to friends. The boys were using the most vulgar language imaginable and when I politely asked if they'd keep their voices down they just laughed. I decided to ignore them. One boy then got out of the pool and yelled at a young child on a nearby playground to "STFU" because the swing he was usuing was squeaking too loudly, he then turned and dove (head first) into 2.5ft of water. When he came up I yelled at him that he could have killed himself and not to do that again. I totally didn't mean to but I was a lifeguard for 10 years and it just happened. The parents said nothing. When is it okay to speak to someone else's kids?