So in The Last Ship an Iowa class battleship shows up fighting for the bad guys.
Now, given the fall of global civilization that came before this point, losing track of a battleship is actually plausible. Could have come loose from its moorings and drifted and been recovered perhaps?
That said, I’d expect the first thing the Navy would do would be to check on which Iowas were accounted for, and prepare a report on any quirks of that unit, such as any mission specific hardware that was installed her sister ships did not receive.
But there has been no mention of this. Just dialog that it’s a battleship, a display claiming ID as an Iowa class, that’s it.
Realistically, given the timeline involved, it’s probably the USS Iowa. The others were museum ships by the time of the show, so probably moored too tightly, and with too much necessary equipment pulled out to be reactivated by anyone save *possibly* the United States. While Iowa would not be easy to reactivate, it might have been possible. And Iowa would have been in San Diego when shit went down, accounting for the Columbian fleet having to traverse the Panama Canal.