How about a 3 kilometer long Pycrete ship? Just keep it in colder waters, virtually torpedo proof. A floating island with just multiple Azipods. You could literally put several Leupold style railway guns on it, or a nuclear power plant for railguns. Could have sub docks. And an airfield long enough for large aircraft.
Why not concrete. Then you wouldnt have to worry about the water temperature.
A battle ship isnt terribly useful if it cant go where the battle is.