Striving to get the maximum cargo carrying capacity and to acquire great economic benefit by use of variations of sea water density in harbours is worth considering by captains and ship operators under the circumstances of sailing safely through shallow waters.

The existent difference between geodetic leveling and tide station is investigated, and this difference comes from the sea level slope, and the sea level slope mechanism caused by ocean current, air pressure and breaking wave is analyzed, the geoid disturbance computed by three dimensional distribution of sea water density is prese.