A Volt is a unit which expresses how much energy a unit of charge will receive (or expend) if it is moved from one location at one voltage to another location at another voltage. Really, it's the difference in voltages from one point to another which is important, just as in other kinds of potential energy (hence the water analogy).
The relationship is Joules = Volts*Coulombs. If you move one Coulomb of negative charge from the negative terminal of a 1.5-volt battery, and put it on the positive terminal, that charge can do 1.5 Joules of work.
I am not sure about your proton and electron example. The electrical potential energy depends quite a lot on exactly where you put your charged particles. The potential energy of two charged objects located a distance r away from each other is proportional to q1*q2/r where q1 is the charge on one object, q2 is the charge on the other, and r is the distance between them. The overall constant of proportionality depends on what units are being used.
If you have two electrons and two protons, then the total energy is the sum over all pairs q1*q2/r, and therefore depends quite a lot on whether you squeeze the protons close to each other (they repel, after all), and similarly for the electrons.
(published on 10/22/2007)