[goodcanadian]

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.

[immibis]

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.