Trout and other fish are more easily caught when the surface of the water is disturbed. Figuratively, ‘to fish in troubled waters’ is to take advantage of other people’s difficulties, particularly in times of general unrest or national emergency. Those who made large profits out of illegally selling rationed goods during World War II were fishing in troubled waters. So is the scoundrel who picks the pockets of a man lying unconscious on the road after being knocked down by a passing car.
