I am of the old school and still like to "dress" on formal nights (there should be 2 on a 7 nite cruise), but like the others I hate to have to pack and carry. Most cruise lines (I've never done carnival) offer advance arrangements for tux rental. Guys can call in their measurements, or sizes, several weeks ahead of departure and a complete tux (including shoes) can be hangining in your cabin locker when you come aboard, just leave it behind when you debark. You can usually make a deal for a fresh shirt for each formal nite, and get different color trimmings, or even a different jacket, for each formal night.
It's all handled by an outfit called Cruiseline Formalwear ( or something like that, I can try to find the URL) and it's reasonable. Last time out, 7 night, got black and white jackets, 2 shirts and all the trimings and it cost about $100. I believe they now carry pieces for the ladies was well.
"There's something about a man in a tuxedo that starts the ladies' hearts a-flutter", said Bahama Mama formally.