The characteristics of laser downlink cross-media transmission through atmosphere, rough sea surface and seawater under three typical weather conditions (clear day, stratus and cirrus) are analyzed by using Monte Carlo simulation method. The results show that, the angular distributions of the light propagating through atmosphere are different under different weather conditions. The divergence is small under clear day condition, which is serious under stratus condition and obvious under cirrus condition. The difference between the spot sizes under clear day and stratus conditions is small, while the size under cirrus condition is about 15% larger. The laser shows different characteristics propagating through atmosphere and sea surface under different weather conditions. The angular divergence is larger and increases with wind speed under clear day condition. It is smaller and decreases with wind speed under stratus condition. The angular divergence changes complicatedly under cirrus condition. Under different weather conditions, there are similar characteristics of light propagating through atmosphere-surface-sea as well as differences. The characteristics of the angular distributions are different in shallow water, while similar in deep water. The spot size changes little with water depth under clear day and stratus conditions, while it decreases as the depth increases under cirrus condition. In addition, the spot center energy attenuation with depth is different under different weather conditions. Under clear day condition the average rate of change is about -0.410 dB/m, while under stratus and cirrus conditions about -0.426 dB/m and -0.413 dB/m respectively. The results could offer a reference for blue-green laser cross-media communication link budget, system design and parameter optimization.