These days formal , informal, casual do not have the same meaning as they used to or as certain cruise lines would like them to have. I've done two Celebrity and a Princess cruise in the past 12 months. You will certainly see people in the prescribed dress
for the evening and you will also see people dress "below"
what the brochure describes. A jacket and a dress shirt, without a tie will not draw a second glance on an informal night on most ships (including the one you are scheduled on).
Relax and enjoy what you are comfortable with, it sounds like you do not intend to show up in cut off denim shorts and a tank top.
I personally wear a jacket to dinner every night (tux or dinner jacket on formal night) but rarely wear a tie with the jacket.