A spotter is someone trained to look for something. An obstacle is something that impedes progress or achievement. Capsizing refers to when a boat or canoe is tipped over until disabled. The act of reversing a capsized canoe is called righting. If a capsized vessel has sufficient flotation to prevent sinking, it may recover on its own if the stability is such that it is not stable inverted.
Translation avoid floating this river if your spotters name is Otis.