I think the brown is from stink weed leaching into the water. I come home with stink weed smell on me some times from there.
I don't think its a cable running WSW. you drive past a pipe above ground that bends down like a knuckle into the ground next to the Sand farm Shed (its also pointing WSW). I think its an intake for water to pump the Silica sand up to the Cement Factory inland.
Can you use your UAV to run this line out to sea and find out. Then I can use my ROV to dive down and we can see what it looks like underwater?

That meat and Potato rock art is pretty funny. I guess the effort some one took to do it just paid off.