I think metallic... the idea is to provide the initial charge to the electrodes from it.. from inside..
when we charge it having a resistivity of low ohms in water and we put a resistive element like the one in the drawing it become clear that a high electric field will exist there at the interface... it not very intuitive
basically it needs a sort of square wave field at the correct frequency and intensity to make the molecules dissociate by collision of the molecules with the freed electrons. Eccles explain this way, the ion transit times is much higher than the free electron speed so it basically needs the chance to hit the ion...
the thing is water has no free electrons its in equilibrium... if we have a reference point that brings the water to very high voltage the electrodes will be at a lower voltage than the reference point because of voltage drop...
so the should result a voltage on the electrodes specially if its connected to a high resistance... it become almost an electrostatic field... my guess is that meyer eccles tayhe haitin all them used this in common... high resistance for having high electrostatic fields...
i posted a document must read,, where it explain surface charges... and it talks about the voltage as field and how it relates to potential difference
a real capacitor is not equal to an ideal capacitor
an ideal capacitor have no fields outside because the charges cancel each others fields inside
a real capacitor have resistance of some sort! so the thing is a real capacitor will indeed have a electrostatic field outside its plates... specially if one of this plates is a layer of water...
water have no free electrons but it has ions... ions attach to electrode because of the free electrons in metal and its ability to move... but they do not attach to a dielectric i guess unless there is a field to force it to happen
my idea is if we have a small electrode just to give the electrostatic charge to the electrodes and restrict from outside we may find that the electrode will be at extremely high voltage potential but current is not allowed to flow on its own...