It grows in mostly intertidal areas that have rocky shores. You see it along the coast of Maine away from open shores the same as you'll see it in Scotland or other Northern coastal areas in Europe. I think it's pretty much limited to the North Atlantic. Slow growing, it attaches to rocky shorelines but can become dislodged by storms, tides, and float off as we're seeing. Rockweed has little float like air pockets along the length of the fronds. These keep it upright while under water(and floating when detached) so it's a very nutrient rich seaweed. A good side to seeing it is that water quality must be good for it to grow. It recedes quickly in polluted water.

I don't think it is the problem for sea water intakes on boats that eel grass is. It's a course frond and as it floats, it stays on the surface. But as Steve says, the mats have collected all kinds of flotsam from plastic bottles to pieces of wood. And as it is being pushed up on the harbor shore all around Rockport, I suspect it may be quite aromatic as it rots in the next few weeks.