We've been on a lot of Carnival cruises and have sailed out of New Orleans, NYC, LA, San Diego, Miami, Seattle, San Francisco, and our own homeport, Port Canaveral. Our shortest cruise has been 7 days and our longest is 15 days. We stay away from summer cruises as much as possible, spring break, and any holiday cruises. I've never seen or heard of a fight onboard on any of our cruises. The only excitement we've witnessed and not planned by Carnival was some moron jumping off the ship in the middle of the day somewhere between San Diego and Hawaii and witnessed by a lot of people (he was brought back on board by the crew).
I think it's time for the cruise lines to get serious about fights. First, hire some big guys to work security. Second, if a fight breaks out, restrict the fighters to their cabins for the rest of the cruise guarded by some of the newly hired big guys, and third, put the clowns on a Do Not Cruise list shared by all the cruise lines.