In a large bowl, add rice and cover with lots of water. Stir rice in water with your hands, then pour off most of the water, and agitate 10 to 15 times by quickly moving your hand back and forth, swishing rice against side of bowl. Rinse the rice several more times, until the water that drains off of the rice is almost clear. Transfer the rice to a colander and let drain for 1 hour, undisturbed. Transfer the rice to a rice cooker, add the water, cover, and cook as directed by manufacturer's directions.
While the rice is cooking, bring rice vinegar to a boil in a small saucepan, and then remove from the heat. Add sugar and salt and stir until dissolved. Allow to cool to room temperature.
When rice is done, allow cooked rice to remain in rice cooker for 10 minutes. Transfer rice to a large shallow mixing bowl (traditionally a wooden tub, called a hangiri), so that rice forms a mound in the center of the bowl. Using a diagonal slicing motion, gently cut into rice with a wooden paddle (called a kijakushi), and pour cooled vinegar mixture over top of rice. "Cut" rice several times to evenly distribute vinegar mixture, then allow to cool. Gently turn rice over from time to time with paddle so that rice cools evenly. You might want to consider using a fan if time is of the essence. When rice has cooled to body temperature, it is ready to use for sushi rolls. Rice can be kept at the proper temperature by placing it in an insulated container such as a small ice chest, covered with a damp kitchen towel.