A simulated oceanic weak turbulence channel in Log-normal distribution and an equal-gain combining receiver were adopted in this paper. Various link parameters were taken into account, including avalanche photodiode (APD) shot noise, channel fading, geometrical loss, Log-normal turbulence, and aperture-averaging factor. On this basis, expressions for the average bit error rate and the upper bound of the average channel capacity of the multiple-input multiple-output (MIMO) underwater wireless optical communication (UWOC) system were theoretically derived. Simultaneously, the influences of the aforementioned parameters on the average bit error rate and average channel capacity of the system were discussed quantitatively. Simulation results show that quadrature amplitude modulation with a proper modulation index, more antennas configured, larger receiving aperture, and higher transmitting power all contribute to better system performance.