I would absolutely talk to a lawyer about whether any activity you witnessed might be criminal conduct for which an NDA would be unenforceable. You may very well save lives.
Carnival has large IT departments monitoring social media, so I will be on their radar here, but I can confirm that there was a lawsuit someone came in touch with me to be on.
Message to the carnival guys monitoring this: Hey guys, I am not coming back, you can't do shit about this, this isn't Facebook. I know you know who I am already, I just don't care.
Love the sentiment, but if you really want to stick it to them, keep your powder dry and coordinate with a legal team about what you should post. Posting too much here could weaken your case if you have one, and justice deserves to be served.
(Obligatory: Not a lawyer, this is not legal advice.)
Message to the carnival guys monitoring this: Hey guys, I am not coming back, you can't do shit about this, this isn't Facebook. I know you know who I am already, I just don't care.