Currently, Water Dragon is only castable on water but continues casting if you substitute off of water.
I think its a strong jutsu, and this may be tough to tweak, but if we remove the ability to continue casting it after getting substituted away from water, we could add in the ability to cast it as long as you're in PROXIMITY to water, kind of like with the Haku needles or ice mirrors. Maybe with a reduce range, something like you have to be within 3 tiles of a water source to cast it.