Marine gravity survey is of great significance to the fields of geophysics research, marine geological mapping, and so on. Usually, accuracy calibration is required since the gravity measurements remain relative at present. For this application, the absolute gravity reference point is necessary, which can be established with classical optical absolute gravimeters. In this article, an absolute gravity benchmark is constructed through a gravity measurement system based on a homemade atomic gravimeter at the quay of Zhoushan Archipelago. Even in the rugged environment of seaboard, a measurement sensitivity of 0.76 mGal

$/\sqrt {\text {Hz}}$

is obtained. Besides, the estimated absolute accuracy of gravity is better than

$30~\mu $

Gal. Interestingly, it is found that the accuracy has been affected greatly by the ocean tidal loading (OTL) effect, especially the absence of an accurate OTL model in the observation region. To further investigate the OTL effect, the continuous absolute gravity observations are carried out for seven days, collecting tidal heights as well. Notably, a peak-to-peak amplitude of approximately

$170~\mu $

Gal is obtained even when the gravity variations caused by solid tide effect have been corrected. However, the calculated values with the existing OTL models seem much smaller than the actual gravity variation data. To solve this problem, an optimized OTL model for offshore gravity measurement has been proposed through an analysis of the existing models. With the improved model of OTL, the calculated results agree well with the experimental data. These findings offer valuable insights for the high-precision calibration of marine gravimeters.

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