Does water level determine the amount of power lost?
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.