[capt. norm]

Not exactly a new idea. The two centuries old Voltaic pile would work just as well with urine as with brine.

What they don't seem to get is the fact that the electrolyte (urine) is not what gets used up. It's the sacrificial metal (zinc in the old timey batteries) that is consumed.

[R. Scott]

Yep. Steel hulled vessels have zinc plates welded to their hull so the zinc will be eroded away by electrolytic corrosion instead of brass fittings in contact with the steel hull. The zinc erodes relatively quickly.