That is how ice powers work in literally every other piece of fictional media ever: they FREEZE the WATER. Ice is SE against Ground because it freezes the WATER in the mud. Ice is SE against Grass because it freezes the WATER in their plant cells. Ice is SE against Flying because it freezes the WATER on their wings. Ice is SE against Dragon because it freezes the WATER in their bodies.

Ice is based entirely around FREEZING WATER. But when up against actual WATER, it jobs? How the FUCK does that work?