Hydrokinesis is the ability to manipulate, create and absorb water as well as wield it in various ways and for various purposes. One with this ability could produce water onto surfaces, project it in blasts or arcs for offensive attack, or even summon tidal waves and tsunamis. In a witches 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.