I know exactly what you mean, I just wear something a little dressier for the informal night and for guys that means a sports jacket. We did notice on our last Royal Caribbean cruise those informal nights, you rarely saw any men in sports jacket, so I think this time, we'll leave that at home.
the difference has become so confusing. I look at it this way, informal is one step-up from casual. If casual for a man means dockers type pants and a polo shirt, then informal to me would be a dressier pant and a button-down collared shirt, maybe a tie, jacket not required. For a woman if casual was a sundress with sandals or capris with sandals, informal would be a nice dress with heels or a nice pair of pants with a nice blouse, maybe silk or a nice skirt/blouse. Hope this helped some.