Rinse the 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 and sea salt; let it cool to room temperature.
Peel and slice the cucumber into thin matchsticks, and slice the avocado into thin wedges.
Thinly slice the raw salmon and yellowfin tuna into uniform sashimi-style strips.
Place a bamboo sushi mat on a flat surface and cover it with plastic wrap.
Lay the nori sheet on the mat and spread the seasoned rice evenly over the surface, leaving a small border at the top.
Carefully flip the nori over so the rice is facing down on the plastic wrap.
Place the cucumber matchsticks in a line across the center of the nori and roll tightly using the bamboo mat.
Drape the sliced salmon, tuna, and avocado over the top of the roll in an alternating rainbow pattern.
Place the plastic wrap back over the roll and give it one final gentle squeeze with the mat to secure the toppings.
Using a very sharp, wet knife, slice the roll into 8 pieces and serve with wasabi paste and coconut aminos.