from my understanding the support material IS water ie the support for the LIQUID resin that is incrementally raised up the print, and as the resin is very slightly less dense than the salt water being used the resin floats on the water, so no extra resin (other than enclosed volumes which will tend to become SOLID resin after curing in sunlight for a period BUT if there is a hole in the bottom of the print then water can enter the enclosed space and result in less resin needed)