Light propagation in a downlink Optical Wireless Communication (OWC) system from above the sea surface to the underwater medium is influenced by wind generated surface waves, associated bubble layer, particulate absorption and scattering, and medium inhomogeneity. The present study aims to model the combined effects of these factors on the channel characteristics, power budget and channel impulse response of the OWC system using Monte-Carlo numerical simulation technique for two different water types (open-oceanic and coastal). The transmission of light across the air-sea interface is modelled by using a Bidirectional Transmittance Distribution Function (BTDF), whereas the medium inhomogeneity is taken into consideration by stratifying the underwater channel based on the in-situ measured IOPs. In order to examine the effect of stratification, the channel characteristics are estimated by considering the medium as homogenous as well as stratified and compared. Comparison results suggest that the medium stratification provides significant improvement in power estimation in comparison to a case where the medium is considered as homogeneous, for all depths. Similarly, the channel impulse responses estimated for these two conditions are notably different in terms of delay spread, suggesting that the medium stratification in clear and moderately turbid waters may be a vital necessity for precise estimation of channel characteristics. These results will be useful for design and implementation of a physical downlink optical wireless communication system. © 2019 Elsevier Ltd