The diffusion Monte Carlo (DMC) method is used to calculate the electron binding energies of two forrns of (H2O)(6)(-). It is found that the DMC method, when using either Hartree-Fock or density functional theory trial wave functions, gives electron binding energies in excellent agreement with the results of large basis set CCSD(T) calculations. This demonstrates that the DMC method will be it viable method for characterizing larger (H2O)(n) ions for which CCSD(T) calculations are not feasible.