Hydro-Cryokinesis

Hydro-Cryokinesis is the elemental ability to freeze water and all things that contain water.

With this power, Aquitarians and Hybrids can cool down and freeze water, from water molecules to the human body. The best known hybrid who has this power is Heather Merrill.

It can be combined with Hydrokinesis, to create a variety of things (ice balls, ice sculptures, etc), but that would eventually melt or break. The hand movement for this power is a stiff "stop" sign, similar to the English sign language for the letter "B". This may symbolize that when Heather is doing this she's slowing down and stopping the water molecules.

If a mermaid with this power is in the Moon Pool during a special planetary alignment on a Full Moon, it is  enhanced to Cryokinesis.