Subs, carriers, and the like would be ok for a long while, supplies are the issue. Nuclear means fuel isn't an issue, just risk of coming ashore for supplies. You'd lose some to taking bitten people onboard, but based on how zombies act in TWD world, they're not patrolling the ocean floor like World War Z, so boats should be ok. WWZ was better in general with showing things go to shit, but at least not having EVERYTHING disintegrate. And maybe it hasn't in TWD, and we're just focused too tightly on our group?
I just finished reading the book World War Z and yes I agree they deal with a zombie Apocalypse in a far more realistic manner and in Z the zombies are fast.
In fact the seat of US government moves to Hawaii and the new "White House," is an aircraft carrier.