Melt margarine in a medium skillet. Add onion, red pepper and garlic. Cook Salt and white pepper. Cover, reduce heat, simmer for 10 minutes. Combine cornstarch and water in a small bowl. Mix well. Add to tomato mixture. Stir until bubbly. Add shrimp and crab and heat to boiling. Reduce heat and simmer for 10 minutes or until seafood is thoroughly cooked. Serve on top of rice. Serves 4.