Pour the crushed crackers into a small bowl. Mix in the melted margarine until evenly coated; set aside.
Stir the broccoli, onion, Cheddar cheese, rice, condensed cream of mushroom soup, and water in 2 quart casserole dish. Sprinkle with the cracker crumbs.
Bake in the preheated oven until the casserole is hot in the center and the top is browned and bubbly, 45 minutes to 1 hour.
