In order for the equilibrium of zinc ions and zinc to be set up, the forward and backward reaction have to occur simultaneously, such that eventually the rate of the forward reaction will be equivalent to the rate of the backward reaction.
It is more energetically favourable for zinc to be oxidised into zinc ions as given by its electrode potential of −0.76V. However, in a single cell, as zinc becomes oxidised, electrons are left behind on the electrode because it has nowhere else to go. It becomes increasingly more difficult for zinc to be oxidised into zinc ions because of the excess number of electrons, which is why zinc doesn't continuously oxidise into zinc ions.
But, for an equilibrum to be established if the zinc electrode were to be submerged into water, zinc ions have to be reduced into zinc, which is less energetically favourable. How does this reaction happen for the equilibrium to be established?