Mission : Into The Deep
Around half-way into the mission you will swim up and inside a light-house. Once you reach the top you will be handed a Proteus missile.
In order to fire the missile, whilst in your hands fire with RT
I thought i had to free aim so took a few seconds to line up what i thought would be a decent hit, but after pressing RT you are looking at a screen and remotely controlling it.
You will notice a large area in the bentre that is glowing Red,Orange,blue,purple. This is where we need to hit. I almost missed this, and nearly hit one of the outer fins so bare this in mind and approach slightly from below the ship.