In a sauce pan, bring water and sugar to a boil, allowing sugar to dissolve.
Run rice in a mesh strainer under water to rinse excess starch off of rice. Add rice to boiling water and cover, reducing heat to a simmer. Simmer for 15 minutes.
After water has evaporated, remove from heat and add vinegar. Fluff rice with a fork, and cover again, allowing rice to steam for an additional 5 minutes.
Meanwhile, quarter and slice a cucumber and halve then slice an avocado. Also, in a small bowl, combine mayonnaise and sriracha.
To Assemble
Divide the rice into two bowls. Top with crab, cucumbers, avocado slices, carrots (optional). Drizzle with sriracha mayo mixture and serve immediately.