There were times in the past four years when the water was still. For days at a time, Michael Phelps didn’t bother to show up at the pool, never rippled the surface. He stayed missing so long that his coach, Bob Bowman, wondered whether they “were going to get this thing done.” But eventually — reluctantly but determinedly — Phelps towed that long torso back into the liquid, where the greatest swimmer of all time belonged.