Combine rice and 6 cups water in a saucepan. Bring to a boil over high heat. Reduce heat to low, cover the pan, and simmer until rice is tender and water has been absorbed, 15 to 20 minutes.

While rice cooks, melt butter in a large skillet over medium heat. Add onion and cook, stirring, until transparent. Stir in garlic and cook until fragrant, about 1 minute.

Stir in the flour until well blended. Gradually stir in the tomato sauce and water, then add the crawfish tails and bring to a simmer. Add the green onions and season with salt, pepper, and Cajun seasoning. Simmer for 5 to 10 minutes over low heat, until the crawfish is cooked but not tough.