My personal approach to formal nights is to just dine elsewhere. Plenty of good calories and reasonable nutrition in the buffets and other places outside the main dining room.

I do respect those who choose to get gussied up. Just not for me. Bears, salmon, whales, glaciers, incredible scenery and Alaska Brewing Company were why I chose the Alaska cruise. Haven't see a bear yet, so will probably have to go back again. Still without formal wear though. (Just carry-on via a small backpack which also lets me skip the whole cruise ship luggage process.)

Incidentally, the buffets were unusually crowded on formal nights. Must be others like me that opt out.