Direct molecular dynamics computation of the absolute entropy for simple point charge model water and ice have been carried out by adiabatic switching processes. A theoretical analysis shows that the average energy for nonadiabatic switching is always larger than the adiabatic energy. Due to the energy dissipation the deviation in entropy is positive for finite rate of switching. Two switching functions are used to examine the computational convergence and accuracy. Also, two reference integrable systems, ideal gas and ideal solid, are proposed as ends of the adiabatic switching. A proper reference system and efficient switching path are necessary for accurate computation of absolute entropy. Systematic and statistical errors of the results are discussed.