Avoiding Carnival is the best solution, apparently. I just googled XXXXX Cruise line brawl, for Carnival, RCCL, NCL, MSC, Holland America. One altercation came up for each of NCL, RCCL, and MSC. None for Holland America or Celebrity. But each Google search came up with several links to brawls on Carnival ships. My favorite was a 60 person brawl on Carnival Magic last July apparently caused by a passenger 3-way followed by cheating complaints. In all likelihood fueled by copious amounts of alcohol.