For me it depends on the cruise. When I do the 3 or 4 night Baja party cruises on Carnival for "Cruise Fancy" night I wear jeans and a long sleeve shirt. On 7 night cruises out of the States I take a suit with me.
When in Europe on Celebrity I took my Tux with a couple of vest and tie sets, 1 white and 1 black Tux shirt and a couple of stud and cuff link sets so I looked different on the 3 formal nights. The European's in the cabin next to us were surprised that I dressed for formal night after only seeing me in jeans the rest of the trip, she said I "Clean up well".
I do not mind dressing up once in a while but not sure I'd want a long cruise where I'd have to be formal most nights.