In a large saute pan over medium high heat, melt the butter. Add the onions, celery, and bell peppers and saute until the vegetables are wilted, about 10 to 12 minutes. Add the crawfish, garlic, and bay leaves and reduce the heat to medium. Cook the crawfish for 10 to 12 minutes, stirring occasionally. Dissolve the flour in the water. Add the crawfish mixture. Season with salt and cayenne. Stir until the mixture thickens, about 4 minutes. Stir in the parsley and green onions and continue cooking for 2 minutes. Serve over steamed rice.