BTW, here is a current discussion (started yesterday) on Cruise Critic on "acceptable" clothing for Holland Gala nights in Alaska:
https://boards.cruisecritic.com/show....php?t=2634431
To the OP, personally, I'd pick out which cruise line I prefer and then search on the Cruise Critic boards for formal night attire for Alaska. You should be able to find quite a bit of discussion around that topic.