Combine cream of mushroom soup, dry onion soup mix, and water in a slow cooker.
Place the pot roast into the slow cooker and make sure to coat it with the soup mixture.
If you want a faster cooking time, set the slow cooker to High and let it cook for 3-4 hours. If you prefer a slower cooking time, set the slow cooker to Low and let it cook for 8-9 hours.