Does water level determine the amount of power lost? In the left image, you can see that Luffy and Brook have their powers drained. However in the right image, both Hancock and Luffy are in standing water but seem to keep their powers.




So, does the amount of power they lose depend on the water level?
Based on what I read and what I remember:


Rain normally doesn't have an effect.
If the Devil Fruit user's body is mostly out of water, then they get weakened a bit.
If the user is totally/mostly in the water, they normally can't move.
Seastone has an effect like that of being totally in water.


The difference of the above pictures: on the left, Luffy and Brook are mostly in water, so they are very weak; on the right, Boa Hancock and Luffy are only standing in knee-deep water, so they don't lose too much power.