A boat carrying steel balls is floating on the surface of water in a tank. If the balls are thrown into the tank one by one, how will it affect the level of water

According to the Archimedes Principle, the balls displace water equal to their mass when they are floating on the boat, but when they are thrown into the tank, they displace water equal to their volume of water. When the ball is only displacing its volume rather than its (greater) mass, less water is displaced because the steel is denser than the water.  Therefore, the level of water will fall if the balls are thrown into the tank.