This area is extremely difficult to reach. Once at the end of the level, enter the Boss fight with Redtail. He will start firing his pistol at you. Dash over and dodge until you reach the end of the small sandbar. Get a running start and jump as far as you can into the water. If you are lucky you will land on a small platform hidden in the water. The platform will take you to a skull and an extra life. Get them both then return to the battle with Redtail.