In a 3-1/2- or 4-quart slow cooker whisk together the water and the soup. Stir in tomatoes, the celery, carrots, onion, garlic, Italian seasoning, salt, and pepper.
Cover and cook on low-heat setting for 7 to 8 hours or high-heat setting for 3-1/2 to 4 hours.
Gently stir in tofu cubes and spaghetti. Sprinkle each serving with cheese.