We investigate the spatial variation of the axial, radial, and tangential diffusion coefficients in carbon nanotubes (CNTs) of various diameters. The effect of confinement and CNT wall on the diffusion coefficient is studied. On the basis of the spatial variation of the diffusion coefficient, the diffusion mechanisms in different regions of the nanotube are identified. The dependence of the diffusion coefficient on the carbon-water interaction parameter is investigated. The average diffusion coefficient in the nanotube as a function of the nanotube diameter is computed, and the diffusion mechanisms, including the transition regimes, are identified. The results are analyzed via hydrogen bonds and water orientations.