Because of its quick cast time and recharge, this is an excellent spell to use on rampaging Warriors or fleeing enemies.
Remember that if it recharges faster than normal, the hex doesn't re-apply itself when it casts on a target already affected by a Water Magic Hex, it only causes damage. This can be important if you need to keep an enemy slowed down a long time.
Unlocking Zhed Shadowhoof after the quest Centaur Blackmail automatically unlocks Freezing Gust. The skill is not available at any Nightfall trainers reached before completing Zhed's quest, and the quest is mandatory for progression. Thus the concept of the "earliest" skill trainer for Freezing Gust is not applicable (any Nightfall trainer will do once Zhed is obtained).