The diffusion Monte Carlo (DMC) method is applied to the water monomer, dime; and hexamer using q-TIP4P/F, one of the most simple empirical water models with flexible monomers. The bias in the time step (Delta tau) and population size (N-w) is investigated. For the binding energies, the bias in Delta tau cancels nearly completely, whereas a noticeable bias in N-w, remains. However, for the isotope shift (e.g, in the dimer binding energies between (H2O)(2) and (D2O)(2)), the systematic errors in N-w do cancel. Consequently, very accurate results for the latter (within,similar to 0.01 kcal/mol) are obtained with moderate numerical effort (N-w similar to 10(3)). For the water hexamer and its (D2O)(6) isotopomer, the DMC results as a function of N-w, are examined for the cage and prism isomers. For a given isomer, the issue of the walker population leaking out of the corresponding basin of attraction is addressed by using appropriate geometric constraints. The population size bias for the hexamer is more severe, and to maintain accuracy similar to that of the dimer, N-w, must be increased by similar to 2 orders of magnitude. Fortunately, when the energy difference between the cage and prism is taken, the biases cancel, thereby reducing the systematic errors to within,similar to 0.01 kcal/mol when using a population of N-w = 4.8 X 10(5) walkers. Consequently, a very accurate result for the isotope shift is also obtained. Notably, both the quantum and isotope effects for the prism-cage energy difference are small.