Because the lack of oxygen and the pressure is too much for most fish/other things. Professional deep diver usually don't go under 400 feet. The deepest any man has gone is James Cameron, he went 35,756 feet (10,898 meters) below the sea level. The deepest point in any ocean is the Challenger Deep in the Marianas Trench it is 36,070 feet (10,994 meters) deep
