Mix beef, crumbs, salt, 1/2-cup buttermilk, and rice. Shape into two dozen meatballs (about 1" thick). In large skillet, brown meatballs in margarine on all sides; pour water over; cover tightly. Boil until rice is tender and meat is cooked (about 30 minutes). Remove from skillet. Mix remaining 1 cup buttermilk with arrowroot to form smooth paste; stir into liquid remaining in skillet. Stirring constantly, cook until gravy is thick (do not boil). Return meatballs to pan and gently heat. Serve with gravy.