Question

why it is difficult to shoot a fish swimming in the water

Answer 1

BECAUSE THE FISH  SWIMMING IN WATER APPEARS TO BE AT LONGER DISTANCE THAN IT IS ACTUALLY TO THE OBSERVER. THIS IS DUE TO REFRACTION OF OF LIGHT RAYS.
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Answer 2

as the relative density of water and air is different when we observe the fish it looks us below the observing point. so, when we shoot at it we must shoot it lower than we see.