What do you wear for a dinner cruise?

sport coat, collared shirt and slacks.

Business casual attire is sufficient. Cruise lines ask guests to refrain from wearing swimwear, flip-flops and the like. Men should wear collared shirts and slacks.
Formal nights are a marketing ploy to get people to dress up and then entice them have semi-professional photos taken so they can sell photo packages. There is no "requirement" to dress formally.

For formal dinners, men wear tuxedos, or a suit and tie at least. Women get away with just about everything but torn jeans .

Check with the cruise company

Your boyfriend ;)

I would wear something of which isn't too formal but it isn't so casual, either. Such as somewhat loose, long plain button shirt with one colour (green, for example) with black pants. Really just anything of which is semi-formal is fine.