Rinse the Japanese Short-Grain Sushi Rice in cold water until the rinse water runs clear, then drain thoroughly.
Combine the rinsed rice with the appropriate amount of Water in a rice cooker or heavy pot and cook according to your rice cooker instructions or until the water is absorbed and the rice is tender.
While the rice cooks, warm Rice Vinegar in a small saucepan with the Sugar and Sea Salt just until the sugar and salt dissolve; do not boil, then let the mixture cool slightly.
Transfer the hot cooked rice to a large, shallow bowl or hangiri; drizzle the vinegar mixture over the rice and fold gently with a paddle in cutting motions to coat each grain while fanning the rice to bring it down to warm, body temperature.
Pat dry and slice the Tuna, Salmon, and Yellowtail against the grain into thin, bite-sized pieces (about 2–3 mm thick and 6–8 cm long).
Prepare the Shrimp by poaching briefly until just cooked, cooling, removing the vein and shell, then butterflying the shrimp so it lies flat over the rice.
Dampen your palms with water to prevent sticking, take a small handful of rice (about the size of a large egg), and shape it into an oblong mound with gentle, firm pressure; do not compress the rice too tightly.
Spread a small smear of Wasabi on one side of a fish slice (or on the rice if you prefer), place the fish over the shaped rice, and press lightly to adhere; repeat to make nigiri with the remaining fish and rice.
For shrimp nigiri, lay the butterflied shrimp over the rice; if desired, secure it with a thin strip of toasted Nori wrapped around the middle.
Arrange the nigiri on a serving platter and offer Soy Sauce and Pickled Ginger on the side for dipping and palate cleansing. Serve immediately for the best texture and flavor.
