I have seen lots of guest dress slightly above casual on a cruise elegant nights. It's no big deal, your vacation. Do what your comfortable with.
I pack two suits. As this is the only time I dress formal. I never thought I'd enjoy it but I really get a kick out of it. The entire attitude of everyone changes that night. Afterwords we ususally dance and play with everyone at the casino bar or Piano bar and see a show. By the time the night is over ,that suit is pretty much done.
Slacks and a decent shirt, maybe a tie is fine. As long as you have fun. The most important part is go how you roll. I've seen jeans , boots and jacket, Ive see jeans shirt and tie,deck shoes, it's all fine.
Ive seen black tie to pricey looking evening gowns , all of it makes the night move with energy. My wife buys designers gowns for around $30 - $40 , and name brand shoes at the thrift stores. Absolutely awesome watching her come down the elevator in the ATRIUM while i sit at the bar waiting for her. Something about those nights cast a spell over me. Especially since I usually spend my summers in shorts t- shirts and flips flops.
Booked 2013 Liberty
" Perfection, is a collection of polished errors"