California Grill (Contemporary Resort) has some good sushi offerings. California Grill Dinner Menu - Contemporary Resort
Tokyo Dining (Epcot's Japan) has sushi, and it was very good. Teppan Edo (also Japan) has Crispy Suklyaki Beef Rolls, which aren't strictly sushi (it's cooked beef rolled up in rice) but oh so good.
Tokyo Dining - Japan Epcot Menus Around Walt Disney World
Teppan Edo - Japan Epcot Menus Around Walt Disney World
Just outside the Kona Cafe (Polynesian Resort) there is a coffee bar that switches to a sushi bar for a few hours during the evening.
Kona Island Sushi Bar - Polynesian Resort
The Swan has Kimonos Sushi Bar, which I've heard good things about but have not had a chance to try yet: Kimonos Sushi Bar Dinner - Swan