Read it through once

'Do you see that long slanting line on the face of the water? Now, that's a reef. Moreover, it's a bluff reef. There is a solid sand-bar under it that is nearly as straight up and down as the side of a house. There is plenty of water close up to it, but mighty little on top of it. If you were to hit it you would knock the boat's brains out. Do you see where the line fringes out at the upper end and begins to fade away?'