In a medium saucepan, bring 375ml (1,1/2 cups) water to a boil. Add rice, and stir. Reduce heat, cover and simmer for 20 minutes until water is absorbed.
In a small bowl, mix the rice vinegar, sugar and salt. Blend the mixture into the rice.
Preheat oven to 150 C. On a medium baking tray, heat nori in the preheated oven 1 to 2 minutes, until warm.
Centre one sheet of nori on a bamboo sushi mat. Wet your hands, spread a thin layer of rice on the sheet of nori, and press into a thin layer. Arrange 1/4 of the cucumber, ginger, avocado and seafood of your choice in a line down the centre of the rice. Lift the end of the mat, and gently roll it over the ingredients, pressing gently. Roll it forward to make a complete roll. Repeat with remaining ingredients.
Cut each roll into 4 to 6 slices using a wet, sharp knife. Serve immediately.