I have been on several cruises and see people dressed up in formal wear. I actually enjoy renting a tux and wearing it to these evenings but that is me. It is something I do not get a chance to do often so I enjoy it. I would say do what makes the evening the best for you. If you like to dress up then go for it. There will be others there that will also be dressed in formal gowns, tuxedos etc. There will also be others that are not dressed so fancy but that is ok too. Please thank your husband for his service to our country. I personally think there is no sharper formal wear than military dress blues. I always enjoy seeing the military personnel when they wear them and am sure others on the cruise will enjoy it too.