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.