Marine gravity measurements are critically affected by ocean tides effect. However, the model of ocean tidal loading (OTL) is not accurate enough for offshore high-precision gravity measurement at present. In this paper, we carry out the investigation of gravity variations for the measured location that is far from the shore. Currently, the OTL model calculates the effect of tidal waves globally. However, the influence of local mass changes on gravity measurement should be taken into account, especially when the observed location is far from the shore. For further verification, the gravity measurement experiments are carried out based on a homemade atomic gravimeter at a location, which is 230 m far away from the shore and surrounded by seawater. Even in the harsh measurement environment of the quay, the continuous monitoring of gravity variation has been done for about three days, and the measurement sensitivity of our atomic gravimeter is estimated to be 0.76 mGal$/\sqrt{\text{Hz}}$ for an interrogation time of 110 ms. Moreover, the gravity changes of approximately 170 $\upmu$Gal have been observed with the variation of regional seawater mass, which agree well with the calculated results based on the new modified model of OTL. The results of this paper may be helpful for the high-precision calibration of marine gravimeters.