Does the ocean purify the water?

That is the only part I feel strange about when going to the beach. When I stand in the water and all the sand is around my feet I feel like something is swimming around my feet and I start to freak out! I can not tell you how many times sea weed has wrapped around my feet and I hadn't known about it being there before hand. I really do freak! haha $-MrDollars-$

I think mountains can purify the water, water from sea that goes in to the rivers, and stuffs but in the ocean - the water is clean in some parts, where the waters are deep... those that are near shore maybe be a little polluted