The authors are working toward practical use of a deterioration diagnosis technique for marine structures based on images shot from a small unmanned boat. This paper reports the effect of the luminance and resolution of images shot from the water surface on the results of deterioration diagnosis of the undersurface of a concrete pier superstructure. Using images with a minimum pixel width of 1-2 mm, a crack width of 1 mm can be detected provided the mean luminance of an image is around 50 to 150. The errors of damage area measurement were around 5%. When the minimum pixel width is 2-4 mm or less, a crack width of 1 mm can be calculated. As to measurement of damage areas, the errors were found to be around 5% when the minimum pixel width of images were 16 mm or less.