If You Look Out Into The Ocean, There Is A Black Line That Separates The Ocean, What Lies Behind The Black Line?

Have you ever been out to sea?  On Atlantic side, the water looks brown and murky.  If you in a dock then is is probably pollution,  On the Pacific side, I was quite surprised the the blue comes to the beack...Than again, I I was in a tourist area like A1A, but blue all of what I saw.  On the Atlantic, I'been way out to sea and once you get out, then is the most beautiful shade of blue Ive ever seen.  I've never seen a black line, only a blue one,and the other side, it is just the sea.