Hi Jolyon and Cedric,
Just a note to consider that colour charts have strongly fluorescing panels. This means that relative to other panels, their appearance in the picture, and their reflectance measured with a spectroradiometer, will be dependent on the illuminant. The pictures below demonstrate this using a SpyderCheckr24 but the same is true for x-rite colorchecker. The first picture uses a blue light source with a yellow filter, the second a green light with a red filter (no white balance on the pictures). The panel at Col 4,Row 3 is particularly fluorescent, but others are also fluorescent to a lesser degree. I have yet to find colour standards that do not fluoresce (including Munsell chips!). This is unlikely to cause any problem for terrestrial pictures, but I can see this influencing the calibration of underwater pictures, especially below a certain depth when using natural illumination (e.g., > 10 m). A simple solution may be to first identify the fluorescent panels and exclude them from the calibration.