Whatever a person insists on seeing before they move is usually the exact thing only their movement could produce.
Certainty demanded up front sets a cautious state, caution keeps the feet out of the water, and the water stays exactly where it was, proof for one more year that crossing was impossible. Flood and harvest arrive in the same season, so whoever waits for the level to drop waits out the only window they had.
The ark moves at the speed of the men carrying it; God's presence does not clear the water before the soles commit. Which sets the order: the option of standing still gets cut off first, and dry ground second. Dry feet keep a person safe on a bank they will never leave.