We present an analysis based on molecular dynamics simulations of water single particle and collective density fluctuations in a protein crystal at 150 and 300 K. For the collective dynamics, the calculations predict the existence of two sound modes. The first one around 35 meV is highly dispersive and the second one around 9 meV is weakly dispersive in the k range studied here (0.5<k<4.2 A(-1)). We provide evidence that the boson peak around 4 meV in the single particle spectra arises from translational motion, is present in the coherent spectra, and is distinct from the two sound modes.