Combine 4 cups water and rice in a saucepan; bring to a boil. Cover; reduce heat to low. Simmer until rice is tender and water has been absorbed, 15 to 20 minutes.
Meanwhile, melt margarine in a large skillet over medium heat. Add onion and bell pepper; cook and stir until tender, 10 to 15 minutes.
Stir in garlic; cook 1 minute.
Add cream of mushroom soup and ½ cup water to skillet; stir to combine. Season with salt, black pepper, and cayenne pepper.
Stir in crawfish tails. Cover and reduce heat to low; simmer 15 to 20 minutes, stirring occasionally. Add water to thin sauce as needed.
Serve hot over rice.
