The Empowered water staff can be found in the dungeons under Daemonheim. It require 50 Magic to wield, and gives infinite water runes, the same as a Staff of Water while equipped. It can occasionally be found in the starting room. It is also a possible drop from the boss, Unholy Cursebearer, and forgotten mages also rarely drop it.
Prior to the Evolution of combat update, this staff could be created by imbuing a bovistrangler staff or better at a runecrafting altar at Runecrafting level 60. At present it is no longer possible to make the staff.