I'm a huge fan of sushi and make my own occasionally. Initially thought it must be free and was very surprised to read the syn values in the book, and brought it up with the Consultant.
Since then I've thought more about it, and there are hidden syns. The fish will be free of course, also the rice. However you have to season sushi rice as it cools down and I forgot about that. One big batch can easily have a couple of tbsp sugar in it, and the mirin (rice cooking wine) may well too. Working out the syn value for a single piece could be tricky though.
Pickled ginger or gari I'd imagine was free, but check he packet to see if they add sugar. Some wasabi paste is synned (goodness knows why, unless they pad it out with flour or something) so check the ingredients if you use that, but the amounts used are so tiny I'd be inclined not to bother synning it - do what works for you.
Nori sheets are also synned, that really irritates me as the book states dried seaweed is free, then the website says in this form they are synned. You're only looking at 1/2 syn per sheet, and some brands are only 5 calories a sheet. It's so healthy and a little goes a very long way, so I wouldn't bother counting them either. I suspect that SW hasn't done much research into the more esoteric areas of free foods. But Japanese cuisine is generally very healthy if you avoid the deep fried stuff!
California rolls often have mayonnaise in them, remember to check for that! And tori kara-age (Japanese KFC to you and I) is deep-fried and delicious, but sometimes used in the thick rolls. Avocado, yes, well spotted