In a 5-to-6-quart slow cooker, place sausage, onion, garlic, celery, and bell pepper. Sprinkle with flour and toss to coat. Add tomatoes and their liquid, 1/2 cup water, and cayenne; season with salt. Cover and cook until vegetables are tender, 3 1/2 hours on high (or 7 hours on low). Add shrimp and okra, cover, and cook until shrimp are opaque throughout, 30 minutes (or 1 hour on low).