Abstract In moderate to strong oceanic turbulence, a differential phase-shift-keying (DPSK) modulated underwater wireless optical communication (UWOC) system with an aperture receiver is presented. The analytical expressions for mean bit error rate (BER) of UWOC system over moderate to strong oceanic turbulence are derived based on Gamma–Gamma channel model by the help of the Whittaker function. The variations of mean BER are investigated versus modulation methods, beam types, aperture diameters as well as four oceanic turbulence parameters, i.e., the rate of dissipation of kinetic energy per unit mass of fluid, the rate of dissipation of mean-squared temperature, the ratio of temperature to salinity contributions to the refractive index spectrum and the kinetic viscosity.