If you light the three drop-down targets just to the right of the Treasure Chest, a magnet is activated that will pull the ball in to the nearby hole. Once there the spear gun begins a sweeping motion and a countdown from 3 starts. If the ball shoots out and hits the targets, you get points. If the target struck is the shark, you get more points! You can control the sweep of the spear gun by activating the flippers. While the gun is moving to the left, you can reverse it by using the left flipper.
The location of the shark before the hunt begins is indicated by a light under the board.
The spear gun can also make it easier to accomplish some missions since it gives you a way of aiming the ball.