When battling the giant Sea Snake be sure to stay locked on. As it comes out if the water lock onto one of it's back plates and blast away at it. In some cases it does nothing but in most the shell will come off. Then follow it to the nearest Manta and get in front of the Manta before the Snake does or else it will gain back health. DO this repeatedly and you will beat it. The hardest part is staying locked on and getting to the Manta before the Snake does. If you lose the Mantas then you lose the level.