This topic seems to come up a lot. You can probably do a search and find several threads. With that said here's my short and sweet on the topic, from a boaters point:
I don't care what you bring with you, as long as it stays in the back of the boat with you.
When it comes to casting, I don't like guys casting past my seat. Why? Cause I don't like people spooking fish I'm trying to catch.
If things aren't going so great, PLEASE throw some suggestions out there. I'm always looking for input from my backseater, in fact I welcome it.
Clean-up your mess before you leave.
DO NOT let me catch you throwing antything but a lure over the side of my boat. I will take you back to the ramp.
I try and treat the non-boater the way I would like to be treated, and keep the boat in a position they can comfortably fish from. Now if I get a non-boaters who's a butt, well I can also make things harder for them.