The idea of a link or a bond Yata theorised is intriguing. How does the water get drained from the body of the Sand Master and into the sand?
So maybe each grain of sand and the algae on it is a lock for investiture. It somehow uses sunlight and converts it into investiture which the Sand Master can release using the water in his body as a key.
A direct application of water as shown by Khriss shows a bright flash of light but when a Sand Master masters sand, the sand glows evenly instead of one huge burst. Since a direct application of water in the physical realm is not effective, I think there may be a cognitive aspect to Sand Mastery. We know that for soulcasting, we manipulate the things in the cognitive realm to affect the physical realm. Maybe this concept can be applied to sand mastery but in a roundabout sort of way. The cognitive aspects of the water and sand are linked somehow by the Sand Masters. When the mastery stops, the water in the cognitive realm returns to the physical realm and coats the sand to make it black.
Hell, to go even further maybe the water itself is invested somehow. I don't have any reasoning to support this so I'm just spouting thoughts.