Find Your Cruise
The boundary between informal and casual is pretty vague. Basically I wear the same thing! A sports jacket is a jacket - like you would wear with chinos - you can buy them in M&S. I guess in the 'old' days a blazer would have been the equivalent. Polo shirt and chinos is fine. I actually have seen very few jackets on informal night - long sleeved shirts and chinos are just fine.