In the long summer months—which in the generally sunny climes where I have lived I regard as May through at least September—I have long made a habit of swimming in as many backyard and club pools as their owners will abide. And since none of the pools are Olympic-sized, I’ve developed a ritual of dipping underwater upon entry, descending to just a few inches from the bottom, and breaststroking from one end to the other, holding my breath for the 15 to 30 seconds various pools might require. It always feels invigorating to pop up to the surface with a…