How did this get water damaged? If the wall is starting to bow out, you've got a larger problem that won't be fixed with a sill replacement. There is probably structural damage that needs to be addressed after you figure out where the water is coming from and develop a plan to prevent this. This is not necessarily a minor problem or an inexpensive fix as it's likely a load-bearing wall. You need to consult with someone to figure out whether this is a problem with the siding, the window, the gutters, the roof, etc. Otherwise you're putting a bandaid on something serious that is likely going to get worse.