Synthetic aperture radar (SAR) is an active instrument which is used to create images of an object. Underwater bottom topography can be retrieved indirectly by measuring variations of the sea surface roughness in the SAR images, although the microwaves cannot penetrate into the water. In this paper, we present a new simple method for bathymetric mapping in the shallow sea. Based on the radiometric correction, sea surface roughness is derived using SAR images. These results are then used for water depth inversions based on the Alpers-Hennings (AH) model, supported by a few true depth data points (sounding data or chart data). This method is used to bathymetric mapping of two areas in the Subei shoal. The study results of the two cases show that the trend of the inversion and true depths match well. The retrieval accuracy depends on the true depth data points. In a case, the true depth data is the sounding data, the relative errors between the inversion and true depths is less than 20%. In the other case, the true depth data is the chart data. And the result is worse, because the measure time of the chart is 1979, and the time of SAR images are 2000s. The bottom topography was changed. The proposed method has two advantages in that it does not require environmental parameters and it is relatively simple to operate.