I don't think Carnival has sushi on board.
I'll tell you one thing, there is another line that is known for having sushi because they made a big deal out of it... well I went and saw it and I could only say "so what?" It wasn't very authentic as it was actually the same ingredients the ship typically has on board anyway, not the really good types of sushi such as octopus or tuna. They had California rolls, and shrimp, and smoked salmon as if they had scooped the ingredients right up from the buffet and rolled them up in rice - yawn. Don't expect great sushi anywhere except for Crystal which is owned by a Japanese company and has a Japanese alternative dining restaurant on board - that is "to die for."