With the technical developments of poisioning and depth detecting, nowadays, the accuracy of sounding results is highly improved than before, while the effect of water correction still stays the same, which makes it become the main restraint for bathymetry precise. It is key to determine tidal time difference between tidal gauges in time-difference method, which is commonly used to conduct water correction. However, the influence of temporal variation of tidal time-difference on water correction is usually ignored, and the time period of observations to determine tidal timedifference is also selected freely. Focusing on these problems, the phenomenon and cause of temporal variation of tidal time-difference are analyzed from the perspective of the merging theory of tidal constituents. Daily determined and hourly determined tidal time differences are compared in value and meaning and the error formula of water correction in time-difference method is educed. Data experiments are conducted in one sea area to show the contrast effect of water correction by using daily and hourly determined time-difference of tide. The research reveals that it is more precise and reasonable to use hourly determined tidal time-difference, than to use daily determined one, which is currently utilized for water correction.