Rinse the rice in cold water, rubbing the grains together to remove excess outer starch. Drain well and place in a pot with 1-1/4 cups cold water. Bring to a boil over high heat, and then turn the heat to its lowest setting, cover and cook until tender, about 15-18 minutes. Combine the vinegar, sugar and salt in a small pot. Bring to a boil and stir to dissolve the sugar. Remove from the heat.
Spoon and spread the cooked rice into a large, shallow-sided pan. Stir in the vinegar mixture; cool the rice to room temperature.
With water moisten hands; shape the rice into twelve oblong balls. Top each with a dot of wasabi, and then drape with a slice of smoked tuna. Set on a platter and serve the sauce alongside for dipping. Serve with wasabi and pickled ginger, if desired.