Most previous works on underwater wireless optical communications (UWOC) assume an ideal horizontal link geometry or a vertical through-the-surface link geometry. The performance of vertical buoy-based surface-to-bottom (downlink) UWOC systems where the source is located on the sea surface has not been studied yet to the best of our knowledge. In practice, the performance of buoy-based downlink UWOC systems will be affected by both the random sea surface slopes caused by wind and scattering property of seawater. In this letter, we take these two factors into account and investigate the bit-error-rate (BER) performance of this downlink UWOC system. We develop a theoretical model to numerically characterize the relationship among transmit power, link range, wind speed, BER and water types for downlink UWOC system. Numerical results suggest the system BER performance degraded by wind speed can be compensated as the turbidity of seawater increases.