Bo of Water

A 6-foot-long staff formed of water springs forth from your hands. The weapon is a quarterstaff and you are proficient with it, dealing 1d6 magical bludgeoning damage on a hit or 1d8 if wielded with two hands plus your Strength or Dexterity modifier. This quarterstaff cannot be targeted by any spell or magical effect that would normally apply to a weapon, such as Shillelagh or Magic Weapon, but can be affected by a monk's Martial Arts feature. 

The bo of water becomes a +1 weapon at 5th level, a +2 at 11th level, and a +3 at 17th level.