The CVA video says that the arc unit is totally sealed and that it burns 5 times hotter than a #209 primer. It demonstrates how virtually all of the powder gets burned when compared to the #209 ignition.
There's a removable breechplug and a wire brush for cleaning it. The arc unit is completely contained within it.
They showed how to swab clean the barrel with solvent, and said that the manual gives instructions for complete takedown & cleaning. But they never mention water besides the waterproof housing for the electronics.
I suppose that they expect people to use non-corrosive powders to alleviate some of the need for water based cleaning. Since all of the powder supposedly burns, there won't be as much fouling. And without a flashhole, whatever residue that's produced is contained within the barrel too.
But the breechplug does look safe to pour some water into, but that's just my guess. There's no where for the water to leak out. So, just pour it out.
The video link is on this page:
http://www.cva.com/products/rifle_electra.htm