Actually most long-range high-voltage transmission lines are DC.
Long-range high-voltage DC (HVDC) interconnects are being discussed in Europe, and have already been widely deployed in China. For local (100km) connections AC is still used almost exclusively, only exception being for some underground cables where induction losses would be too high.
https://upload.wikimedia.org/wikipedia/commons/6/6d/Electric...
Additionally the capacitive losses of an undersea HVAC cable are prohibitive, leaving only HVDC as an option.