When you drink far more than your kidneys can clear, the sodium in your blood gets diluted: that's hyponatraemia. Water then moves into cells, including brain cells, which can cause nausea, confusion and even seizures. Marathon runners and people in drinking contests have been harmed this way. During exercise, drink according to thirst.
Chlorine on its own is fairly gentle. But when it combines with urine, sweat and other grime from swimmers, it forms chloramines, which cause stinging eyes and that classic 'chlorine smell'. A well-kept pool barely smells at all. Shower before you swim and never pee in the water.
Muscle mass slowly shrinks with age, putting independence at risk. But in 1990, a study had nursing home residents aged 86 to 96 train their legs for eight weeks: their strength more than doubled. Muscle responds to training at any age. Getting up from a chair without using your hands is already an exercise.