You control water in order to maintain your missing arm. This requires you to concentrate on spell "Shape Water" (this overwrites basic version of this spell as a "Discovery" from Hermit's background) in order to use and mentain this arm (all concentration properties are applied). You can use it to attack with it or to make basic uses of fingerless arm (it behaves like a water-shaped tentackle). While active, it can be used as basic shield, although some effects can ignore it.