The diffuse attenuation coefficient Kd is one of the most common metrics for quantifying water turbidity and the penetration depth of solar radiation. Recently, Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) has demonstrated its ability to estimate Kd using water column decay profiles. However, this method becomes challenging in the daytime and nearshore areas, where the water column backscattered signals are contaminated by the background noise and afterpulses. In this study, we have proposed a novel algorithm to retrieve the diffuse attenuation coefficient using ICESat-2 bathymetric information (i.e., seabed reflected signals). Because the seabed signal level is much stronger than that from the water column, a significant advantage of this method is the greater noise immunity. The performance is validated against the MODIS ocean color measurements with mean relative differences (MRDs) of<32% using both daytime and nighttime ICESat-2 data in six sea and large lake nearshore areas.