Large ships have expansion joints in the and they also have built in certain amount of flexibility. An rigid hull would either Hog or sag depending if it only on one wave or caught between 2 waves. Some of you might remember those super tankers that would break up going around Cape Hope back in the 70's and early 80's . That was due to sagging and the keel would snap. Ships where just too rigid.
The framing it what give the hull it strength not the outer plating Most ships plating is just 1/4 inch steel. War ships are built little thicker and in the armor belt area steel can be up to 12 inches thick.
John