The power to control, generate and/or absorb water and moisture.
Hydrokinesis is the ability to manipulate water in various ways. It is a common elemental power possessed by witches. Generating water on the other hand is hydrogenesis.
Hydrokinesis is the ability to manipulate water and wield it in various ways and for various purposes. One with this ability could control bodies of water and bend it to his/her will. Creating water out of thin air instead of pulling water from an existing body is a hydrogenetic ability. In a witch's case, it is a naturally occuring and common power, and does not necessarily require a spell to use, similar to telekinesis.
The power to manipulate and create ice is a variation and sub-element of water. One with this power could create, harness, and absorb ice in various ways.