I have seen sushi on the dream all the way aft past the meat carving station as recently as February this year. I do not know if it is there every day since I don't eat at cabanas every day.
There is generally some type of fish dish available every night in the MDRs. There are current menus available on the Disney Cruiseline Blog run by Scott. In addition, you can always make a request to your server and a special meal may be able to be prepared for you, but they tend to require at least 24 hours notice, so your first day you may not have a special tuna dish.