Bob, cruise lines try to establish an atmosphere of a fine hotel or resort in the evenings and shorts are frowned upon even after diner. But, No one will stop you from wearing them. If you change, you probably won't be alone. The majority of guys will probably wear docker style pants with collared golf style shirts. Don't forget the public rooms are highly airconditioned and even though it may have been warm while in port, on a moving ship, it can be quite cool on deck.