From what I know about coastal tribes, a modified pinch was correct.
For a modified pinch, you use the pinch, with the middle and ring finger on the string helping pull. However, the grip you used in the picture would work for a draw to the chest. Bowfishing bows were not as heavy draw as war or hunting bows from my experience with the Seminoles. I fully reserve the right to be wrong about this specific tribe, but I have at least been bowfishing with Seminoles.