I don't know what you say to someone who is silting everything up. I know my original dive instructor told me I was traumatizing the anemones . . . but he didn't teach any other kicks. He did want me to stop diving standing up, though 
I run into the same situation when I see someone riding badly. Generally, I either don't say anything, unless I know them or know they are novices. Then I'll ask, "Would you like any pointers?" or "May I offer some ideas?" Amazingly enough, many people leap at the chance to get some help and do better, especially if information is offered in a non-threatening and non-judgmental way.

I run into the same situation when I see someone riding badly. Generally, I either don't say anything, unless I know them or know they are novices. Then I'll ask, "Would you like any pointers?" or "May I offer some ideas?" Amazingly enough, many people leap at the chance to get some help and do better, especially if information is offered in a non-threatening and non-judgmental way.