1. In a small bowl, combine 2 tablespoons of the vinegar with the sugar, stirring until dissolved. In a medium saucepan, combine the rice with the water and bring to a boil. Stir, then cover and cook over low heat until the liquid is absorbed, about 15 minutes. Remove from the heat and let stand, covered, for 10 minutes. Spread the rice on a large platter and toss with the sweetened vinegar. Refrigerate briefly until cool.
2. Bring a large pot of water to a boil. Add the lobster and cook for 8 minutes. Remove the meat from the tail and claws. Discard the intestine in the tail. Coarsely chop the meat and let cool.
3. Toast the sesame seeds in a small dry skillet over moderate heat, stirring constantly, until golden, about 3 minutes. Transfer to a plate to cool.
4. In a large bowl, combine the remaining 2 tablespoons of rice vinegar with the oil, ginger, wasabi paste and soy sauce. Season with the sea salt. Add the rice, radishes, scallions, cucumber and lobster meat and toss gently to combine; let stand for 15 minutes to blend the flavors. Fold in the avocado, sesame seeds and nori just before serving.
Make Ahead: The lobster, sesame seeds and dressing can all be prepared up to 1 day ahead.
Wine Recommendation: Toasted sesame seeds, sweet sushi rice and mild lobster make this dish a good match for a crisp, aromatic Australian Riesling, such as the 1997 Pikes or the 1998 Grosset Polish Hill.