DEAR MISS MANNERS: I belong to a club that has an indoor and outdoor pool, so I enjoy swimming year-round. An older gentleman also swims there, always at the same time I do. Because of work and other responsibilities, I am limited to swimming at that time of day. This gentleman either has some sort of medical issue or is taking a medication that makes him cough — loudly and constantly. His laps are punctuated at each end of the pool by relentless, loud, dry coughing. It’s not the kind that doubles him over or stops him from swimming; it’s a regular, rapid, rhythmic, dry, barking cough that go…