Helmet of Fin Gleam
I know others may have submited this but this is more detailed.
First travel to anvil (far west of the world map) and west on the coast you'll find an island . Around the middle of it you'll find a rock . From there look around West-ish, out to sea . Walk forward and swim down into the bottom and follow the ground until you reach a skeleton . The helmet is there. Its powers are night vision, water breathing and detect life 20 ft.
Still don't understand look up a video for it.
Edited: Apr 14 2011
Added by: SinisterKiddo Apr 9th 2011, ID#7202