Melt butter in a large skillet over medium heat. Add onion and next 3 ingredients; sauté, stirring constantly, 5 minutes.
Stir in crawfish and next 5 ingredients; cook 5 minutes. Stir in flour; cook, stirring constantly, 2 minutes.
Stir in water gradually; cook over low heat 20 minutes, stirring mixture occasionally.
Stir in green onions and parsley; cook 3 minutes. Serve over rice.