Serve this sushi cake as a precursor to a light Asian-style meal, or alone with hot or cold sake. Since no raw fish is used -- only cooked shrimp and lump crabmeat -- the cake can be made ahead of time and chilled until ready to serve.
In a small bowl, whisk together vinegar, salt, and sugar; set aside. Place the water and rice in a large saucepan; bring to a boil over high heat. Reduce heat to low; simmer, covered, until rice has absorbed all water, about 15 minutes.
Transfer rice to a large bowl. Stirring constantly, slowly add reserved vinegar mixture until fully incorporated. Let cool completely, stirring occasionally.
In a medium bowl, whisk together mayonnaise, chives, two teaspoons wasabi mixture, and lemon juice; season with salt and pepper. Let stand 10 minutes at room temperature. Transfer to a nonreactive bowl, and add crab, shrimp, and one tablespoon ginger. Mix well to combine.
Toast nori sheets to enhance their flavor: Using tongs, wave each sheet over a gas burner or Sterno flame, flipping and turning until crisp and darkened in color, 30 to 60 seconds; both changes are subtle, so watch carefully.
Assemble cake: Place a nori sheet, shiny side down, on a dry work surface, and cover, using moistened fingers, with 2 1/2 cups rice. Cover with plastic wrap, level with rolling pin (remove plastic wrap), and brush with wasabi mixture. Place a nori sheet on top, and spread with one cup rice. Level with a rolling pin, and brush rice with wasabi mixture. Arrange crab mixture evenly on top; sprinkle with remaining tablespoon ginger. Spread with one cup rice. Place remaining nori sheet on top; cover with 2 1/2 cups rice, and level cake with rolling pin. Trim edges, using a wet and sharp knife, and transfer cake to a plate. Cover with plastic wrap; refrigerate at least two hours or overnight.
To serve, slice cake crosswise into three equal pieces. Place on a platter, and garnish with pickled ginger, radish sprouts, shrimp, and toasted sesame seeds. Serve with wasabi mayonnaise.
Wasabi, nori, and pickled ginger can be found in the Asian-food section of some supermarkets.