The dynamics of liquid water is investigated theoretically on the molecular length scale. It is shown that there exist local collective motions of water molecules and fluctuation associated with hydrogen bond rearrangement dynamics. The nature of the collective motions (time scale, spatial scale, and energy scale) is analyzed by examining potential energy surfaces involved. It is shown that the dynamics of liquid water involves multirelaxation processes, and certain physical quantities yield an inverse relationship between the power spectra and the frequency. These results of liquid water are compared with those of water clusters.