Rinse sushi rice under cold water until the water runs clear, then cook according to package instructions.
While the rice is still warm, gently fold in the rice vinegar, maple syrup, and sea salt to season.
Using a very sharp knife, slice the sushi-grade salmon and ahi tuna into uniform 1/4-inch thick pieces across the grain.
Slice the cucumber into thin matchsticks and the avocado into thin wedges.
To make nigiri, wet your hands and form small oblong mounds of rice, then drape a slice of fish over each mound.
To make sashimi, arrange the remaining fish slices elegantly on a platter without rice.
Cut the nori into strips to wrap around rice mounds or use as a base for small hand-rolls with cucumber and avocado.
Serve the platter immediately with tamari for dipping, wasabi paste, and pickled ginger on the side.