An undersea cable is made up of a about a dozen different layers of alternating conductors, braided jackets, and water-proofing. Power is provided by a conductor surrounding the fiber in center of the cable.

http://en.wikipedia.org/wiki/Submarine_communications_cable

Usually at the landing sites there's power feed equipment, as explained at the end of the article. There's some more basic information at http://en.wikipedia.org/wiki/Submarine_communications_cable