HELP! STUCK! First game (modified for XBox)

I'm stuck in Mission 12-ish.
I understand the controls and but the underwater gun isn't working.
I don't even think I have it, was I supposed to collect it on the way? What gun is it? I have Ebony & Ivory, Shot Guns, Grenade Gun and another gun I can't remember at the moment - I think it's a rifle.
I also have the electric sword, Rebellion and the brand fist thing.
Plleeaassee answer I'm totally desperate!
I will actually sell my soul for the answer! Please help me.