The diffuse attenuation coefficient is an essential parameter of a semi-analytical model of radiances used to process a remotely-sensed image for bottom mapping of coral reefs in French Polynesia. Spectral and wide-band values (SPOT-XS-1 and XS-2 and Landsat TM-1) or the diffuse attenuation coefficient were computed fromin situspectral irradiances measured with an underwater spectroradiometer in the ocean and in the lagoonal waters of two islands in French Polynesia. These data show that emerged zones may strongly influence attenuation, mainly at short wavelengths. Attenuation was also assessed statistically in two different ways from a SPOT image of the island of Bora Bora. This image was then processed to obtain a map of the bottom corrected for water column effects.