Be proud that these people all have the right to speak their mind. You don't have to browse the internet to find bad attitudes....you'll meet them personally at any resort in any country.
There are 350 million Americans (give or take a million here or there) and there's bound to be a few wise-asses.
I'm Canadian and travel a lot. I NEVER wear a flag, and have been treated great because of who I am....not because of where I'm from. I look just as American as anyone (if that's even possible if you know what I mean).
And remember this, when your a super-power who stands up to the bullies of the world (really....who else takes them on) your bound to make a few people jealous.
Enjoy the cruises, crew and friends and let the rest go. Misery loves company.
(hmmmmm.......shoulda been a preacher)