You will need plugin filter named Flood, which can be found here.

1. Open any image up.

2. Use your Selection tool and select part of your image.  You will need to play around with this to see just how much water you want in your image.  Try selecting a small selection of your image first and if you want me, just undo and try again until you get what you want.

3. Go to your plugins > flood

Click ok.. and this is what you get..