Rinse sushi rice in a sieve until water is clear.
Let rice drain briefly and place rice and 500 ml (approximately 2 cups) water in a pot and cover and cook over low heat for about 25 minutes. Remove from heat and let stand. Place a towel over pot and place lid over towel.
Rinse cucumber, drain and wipe dry and cut in half lengthwise and remove seeds with a spoon. Cut cucumber halves into 1 cm (approximately 1/3 inch) strips.
Rinse fish and pat dry with paper towel.
For rice marinade, boil vinegar, sugar, 1 teaspoon salt and kombu. Pour sauce through a sieve and let cool.
For the sushi sauce, boil soy sauce, 50 ml (approximately 1/4 cup) water and bonito flakes. Pour sauce through a sieve and let cool. Cut fish fillets into thin strips.
Place cooled rice in a bowl and gently stir rice marinade into rice.
Place 1 sheet nori on each bamboo mat and spread a layer of rice about 5mm (approximately 1/4 inch) thick over lower third of nori sheet. Press a small groove in the middle of sushi rice and brush with a thin layer of wasabi. Arrange cucumber, salmon or tuna strips in groove. Lift and roll bamboo mat, compressing ingredients into a cylinder. Remove mat. With a very sharp knife, cut rolls into equal pieces. Repeat steps to prepare more rolls. Serve with sushi sauce and pickled ginger.