On one hand, it sticks to him despite being a very thin layer that physics says his body heat alone would evaporate.
On the other hand, he only used it and walked slowly, instead of running or flying higher in the air.
Seems to be a limits matter.
Long enough for the kunai to reach the target, think.
We have already seen it done with the lightning element.
If not, what about special kunai made with energy manipulating properties such as those of Asuma?
Indeed.
He would simply (imo) cover the weapons in the same layer of water he covers himself in to bend light.
The idea of the water being too thin to handle major movement is opposed by it being controlled as well as the fact that normal wind would easily evaporate it if that physics applied...