The scintillation index of plane wave propagation in anisotropic underwater turbulence under moderate-to-strong turbulent conditions is analyzed in this paper. A closed-form expression for the average channel capacity of underwater wireless optical communication (UWOC) systems is also proposed based on the Málaga fading model. The newly derived capacity model is effective in evaluating the influence of the link distance, the wavelength, the receiving aperture diameter, the anisotropic factor, the dissipation rate of mean squared temperature, and the eddy diffusivity ratio on the performance of these systems. Simulation results show that applying a large receiving aperture diameter and wavelength can improve the UWOC quality significantly. The contributions of anisotropy, temperature, and salinity also need to be considered in practical UWOC applications. The results reported in this paper will be helpful to researchers designing UWOC systems.