Waterlogue offers water color filters for photos. The results are impressive. However, for my taste, the app is a bit slow and the interaction is limited. I would prefer more influence on the pictures while they are painted by the algorithm.
Get it while it is on sale for 99c.
Popsicolor is a photo filter app to create water color half tone images. You have to choose a pair of colors and a few other settings to turn your photos into something special. In the works above: sophia two-tone
Only caveat: use the latest hardware b/c rendering is not the fastest for playful experimenting.
Popsicolor | my gallery of Popsicolor images