Underwater white balance and colour correction might be a bigger task than simple 18%gray… The problem is that water doesn’t transmit the light in the same way the air does, so you need full color checker along with dedicated algo for color correction if you want to have “real colors”
I’m currently using a slate, which is white and it does the job pretty well. However, white adds a bit too much exposure, when recording in log. Anyway I might give it a try with the light grey PLA and see how it goes. Thanks