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.