“Being pool-trained, I'm used to seeing four sides and a bottom. When that clarity is removed I get nervous. I imagine things. Sharks, the slippery sides of large fish, shaggy pieces of sunken frigates, dark corroded iron, currents. I can swim along the shore, my usual stroke rolled and tipped by the waves, the ribbed sandy bottom wiggling beneath me, but eventually I get spooked by the open-ended horizon, the cloudy blue thought of that sheer drop-the continental shelf.”