First off, Pete, if you are still following this thread- you will be fine. My dh did not wear a jacket, just a nice shirt with slacks and a tie on our more formal night (Tritons) and he was more than fine.
We did the 4 night, so there was really no "formal" night. But we changed for dinner each night. Typically the boys and my dh wore golf shirts and khakis and I work a summer dress. It was a rare opportunity for us to all look nice at the
same time.
I'm answering just to offer a different perspective (I think). A week before our wedding I bought dh new walking shorts, t-shirts, pants and golf shirts for our honeymoon cruise (eons ago). He was adament that he wasn't a preppy guy and was really quite offended that I was getting him a new wardrobe. Said his jean cut-offs and tanks would be fine for the cruise. He was VERY thankful

after we got on board that I had packed the new items as he would have been out of place and he would have felt awful. Now if you don't care what people may or may not say, then do what you want. We personally like to "fit in" to the surroundings, neither being the most dressed up nor the most dressed down. That enables us both to enjoy our surroundings with one less thing to worry about.
To the poster whose partner was planning on wearing a tux or tie, just be prepared for any little one's innocent comment.

I know my guys would quite loudly ask why the girl grown up is wearing boy clothes. Kids can and do say what is on their mind. So just be prepared. And if it is mine that asks I'll explain
again that different people are comfortable in different clothes.
