Capt Cooks and Sunshine Seasons have the same type of sushi trays you'd find in a grocery store - prepackaged single rolls, typically California roll & spicy tuna roll - and they do count as an entree on the dining plan, so it makes for a light meal.
California Grill's sushi is mostly appetizers. The appetizers are single rolls, but they are much larger in size than grocery store/counter service prepackaged type rolls. The Snake in the Grass and Yoshi's Sampler are entrees. I haven't seen them personally, but I have to assume that they're counted as entrees because they're larger.
Tokyo Dining is another option for sushi. Reports are that they offer a couple of sushi combination platters and some mixed entrees that include some sushi.
If you're on the deluxe plan, there is also sushi on the appetizer menu at Kona Cafe.