Rinse the rice under cold water until clear. Combine with water in a pot, bring to a boil, then cover and simmer on low for 20 minutes.
Whisk rice vinegar, sugar, and salt together (microwave for 30 seconds to help dissolve). Fold into cooked rice and let cool.
Chop cucumber and avocado into bite-sized cubes. Shred the crab meat.
Mix mayonnaise, sriracha, and soy sauce in a small bowl until smooth.
Assemble bowls by adding rice, then arranging crab, cucumber, and avocado on top. Sprinkle with sesame seeds.
Store in airtight containers for up to 4 days. Keep sauce separate until serving.