Thanks for your comments, guys. The black dot is a buoy, and at a larger image size can be distinguished as such. I've considered taking it out, and maybe I will, because it does tend to catch the eye and make one wonder whether it's just some crud. The reason for possibly leaving it in: when large enough to see as a buoy, to me it represents the exit to the (safe) harbor, beyond which lies a stormy sea. But maybe it's too small to serve that purpose.