Thats probably the key problem with manual WB underwater: You get the balance right at the distance you're putting the card in front of your lense (in most cases 20-30cm), but the color temperature at your target, which swim at a longer distance away will be different again.   In Thani's example, his skin-tones and the chart look great, but the sand two meters down looks blueish. However, overall I like Thani's white balance in his Raja Ampat clip filmed on the FS7.