I need to calculate the redox potential for the following:

Question

1/2 O2 + 2H+ + 2e- <-> H2O

I'm using the formula

E = E'_0 + RT\nF ln ([electron acceptor]/[electron donor])

The electron acceptor is H+, so I can use the concentration of that which is indicated by the pH.

However, the electron donor seems to be H2O. The molarity of pure water is 55.346 M. Am I really supposed to use that? I have a suspicion that I'm not supposed to do that.