Why modern cruise ships make poor wartime transports

Could today's large passenger ships serve in war as liners did during the world wars?

Generally not: modern cruise ships are slower and less conversion-friendly, although Queen Mary 2 is a notable possible exception.

The passenger ships mobilized in the world wars belonged to a different design era, when fast liners combined speed with forms that could be adapted for wartime transport. Most modern cruise ships are slower and harder to convert for military use. Their large accommodation capacity does not by itself make them efficient troop or equipment carriers under combat conditions. Queen Mary 2 is a possible exception because her speed and more liner-like hull preserve some of the qualities that made earlier passenger ships strategically useful.