It doesn't seem like anyone directly answered your question. As far as I am aware, all long distance undersea power cables are high voltage DC. I believe this has to do with the efficiency of power transfer over long distances.
AC loses power by inductively and capacitively coupling to nearby objects. It's manageable at medium distances above ground, cheaper than a pair of converter stations. However, water is much more conductive than air and losses from an underwater AC cable would be much greater.
