Clear two veteran levels and ??? and push the joypad left as if you were trying to select a different battlefield. The highlight will disappear and then you press A or B you will play on one of the two player levels.
During the cannon stage, if you click five times on a place where the cannon can't be built, you'll go on ahead to the firing stage. One strategy is to place the cannon over a boat you plan to target and click five times, so that when you start the firing stage the crosshairs will already be over he boat.