To prepare rice, rinse rice thoroughly in a sieve. Drain well. Bring 2 cups water to a boil in a medium saucepan; add rice and salt. Cover, reduce heat, and simmer 20 minutes or until liquid is absorbed. Remove from heat; uncover and cool to room temperature.
To prepare dressing, combine vinegar and next 5 ingredients (vinegar through garlic) in a small bowl. Add wasabi, if desired. Combine cooled rice, dressing, cucumber, onion, and sesame seeds in a large bowl. Sprinkle evenly with nori.