I think Carnival is geared more to appeal to family orientated couples. Given that I could hear the TV in the next cabin, I don't think there would be much privacy for such activity. Give me a nice elderly couple for neighbors, so I can sleep at night. I want to be ready to go in the mornings, to get all I can out of the days activities.
You might want to check out the "adult only " cruises on Celebrity cruiselines, like the New Years cruise. I believe it's a Mexican Riviera cruise. Just a wild guess. Sometimes things get pretty wild on Windjammer cruises, but I think they are primarily singles.
Oh, it just occurred to me that you might mean the more risque interpretation of the word "swingers". If that is the case, I don't think you're going too successful on cruise ships, not even the "adult only " Celebrity cruise. You would probably do better looking at a resort like "Hedonism", instead of a cruise.