Conversations like this always devolve into back and forth sniping, but I will throw my two cents in for what it's worth...
I am a big believer in adhering to what is expected behavior. I used to hate following the rules, but now I've learned they are for everyone's enjoyment. I never lived through the days of dressing fancy for flights and cruises, but I kind of wish things were still that way.
However, people spend good amounts of money to go on these trips, so I think they should make the vacation their own. You should make the vacation your own. Wear what you want within reason, and just enjoy. We are all there to have a fun, relaxing time. If you are getting caught up with what other people are doing/wearing/saying/etc. YOU are NOT enjoying YOUR vacation.
So, hope for the best in humanity, but be tolerant when people don't live up to your specific standards.