Combine the rice and water in a saucepan, and bring to a boil. Cover, and reduce heat to low. Simmer for 15 to 20 minutes, until rice is tender and water has been absorbed.
While the rice is cooking, melt the butter in a large skillet over medium heat. Add the onion, and saute until transparent. Stir in the garlic, and cook for a 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. Serve over hot cooked rice.