Hong Kong's waters are characterised by the interaction of oceanic and estuarine water masses, which vary in relative effects throughout the year. The variable freshwater discharge from the Pearl River has a marked influence on Hong Kong waters.

During the summer, an oceanic flow from the south-west to the north-east brings the warm, high-salinity water of the Hainan Current into Hong Kong waters. This interacts with fresh water from the Pearl River and divides Hong Kong into three distinct zones. In the west, where the fresh water influence is greatest, the environment is estuarine and the water is brackish. In the east, the water is mainly oceanic with relatively minor dilution from rainfall and runoff from streams. The limits of the central transitional zone vary depending upon the relative influence of Pearl River water and marine currents.

During the winter dry season, the Kuroshio oceanic current brings warm water of high salinity from the Pacific through the Luzon Strait. The freshwater discharge of the Pearl River is much lower than in the summer and salinity is more uniform across Hong Kong. The coastal Taiwan current also brings cold water from the north-east down the South China coast, affecting inshore waters.

The normal tidal range in Hong Kong waters is between one and two metres, depending on the relative influence of the moon and sun. The tidal pattern is complex due to the relative effects of the diurnal and semi-diurnal components. The basic pattern during flood tides is for oceanic water to flow north into Mirs Bay and west through Lei Yue Mun into Victoria Harbour and through Kap Shui Mun and the Ma Wan Channel. This flow is reversed during the ebb tide. Maximum tidal currents generally range from 0.5 to 1.5 knots, peaking at up to five knots in narrow channels.