So why do battery chargers need two prongs? Why not just one?
The water/hydraulic analogy for electricity is OK but honestly not that great given how popular it is and this is one of the big areas where it fails.  

When you charge a battery you aren't filling it up with charge like a bucket.  The battery stays electrically net neutral. By charging it you are driving a chemical reaction between ions in the electrolyte and the electrodes.  If you had a capacitor instead of a battery you would be building up equal and opposite charges on the plates, but the net charge would be zero.

Current always flows in a loop (Kirchhoff's law) so if you want to think about water pressure consider a closed hydraulic system where you have a pump pushing water out at high pressure and receiving it back at low pressure.  Loads can extract work from the fluid due to the pressure difference but they don't store material quantities.