Rinse rice under cold running water until it runs clear.

Bring water and rice to a boil in a large saucepan. Reduce heat to a simmer; simmer for 10 minutes. Turn off heat, let rice sit without opening the lid for 20 minutes; the remaining steam continues to cook the rice.

Stir rice wine vinegar and sugar together in a small bowl until sugar mostly dissolved; stir into rice. Cool rice, 15 to 20 minutes.

Chop yellowtail into 1/4-inch cubes; add to a bowl. Stir in mayonnaise and Sriracha; set aside. Cover a bamboo sushi rolling mat with plastic wrap; set aside.

Lay 1 nori sheet on a work surface, shiny-side down. Place 1/4 rice on nori, spread evenly from edge to edge, about 1/4-inch thick. Flip nori onto the prepared plastic-covered mat. Place 1/4 each yellowtail mixture and cucumber in a line along one short edge.

Roll rice and nori sheet around filling, using the bamboo mat to help. Press roll tightly to form a uniform log. Spread 1/2 tablespoon tobiko in a long thin line across top of roll. Press roll with bamboo mat. Repeat with remaining nori sheets, rice, yellowtail mixture, cucumber, and tobiko.