By using the Gauss-Hermite quadrature integration approximation,the analytical expressions of unconditional bit error rate (BER) for the single-input single- output (SISO),single-input multiple-output (SIMO) and multiple-input multiple-output (MIMO) underwater w ireless optical communication (UWOC) systems with binary phase shift keying (BPSK) modulation sc heme in weak oceanic turbulence which is modeled as log-normal distribution are derived.T he effects of three main oceanic parameters (i.e.,the rate of dissipation of the mean squared tempe rature,the rate of dissipation of the turbulent kinetic energy,and the ratio of temperature to salinity contribution to the refractive index spectrum) and the propagation distance on t he UWOC system performance for spherical wave are investigated.The simulated results show that the unconditional BER performance improves with a smaller rate of dissipation o f the mean squared temperature,ratio of temperature to salinity contribution to the refractive index spectrum, propagation distance or a larger rate of dissipation of the turbulent kinetic en ergy.Besides, the system performances with different transmitter or receiver numbers under the same oceanic turbulence parameters are also simulated.The results indicate that the system p erformance could be improved when the UWOC system adopting MIMO technology.