I got off a cruise a bit over two weeks ago and all three of us caught it. All of our symptoms were considered "mild" (we're vaxxed and boosted), but it still wasn't a pleasant experience, I'm just now getting back to normal.
My daughter had a tracking app on her phone and she had at least 6 notices of close encounters with others who had tested positive while on the cruise. In hindsight I wish we'd have masked up while in shows, clubs, elevators, etc.
As far as any changes to vaccination policy, anything will just be pure speculation until Carnival makes an announcement.