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.
Do you happen to have the spectral reflectance chart for SpyderCheckr24? Could you share it with me?