What Carnival is actually like, and who it suits
Carnival's reputation is about ten years out of date, in both directions.
The "party ship" thing is overstated — on a seven-night Caribbean sailing you will find far more families and grandparents than you will spring breakers. But the shorter runs, three and four nights out of Florida, genuinely are louder, younger and drunker, and that is where the reputation keeps getting renewed. The itinerary length tells you more about the crowd than the ship does.
What Carnival is unambiguously good at is price. You get a lot of ship for the money, the food is better than the snobbery suggests, and the crew are usually the friendliest afloat.
What it is not: quiet, refined, or subtle. If your ideal evening is a string quartet, book elsewhere and be happier.
Anyone done both a three-night and a week on the same ship? The difference in crowd is the useful data point.
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