Ice Water is a Double Tech in Chrono Trigger used by Marle and Frog. It's learned once both characters learn Ice and Water respectively. Deals Water damage to all enemies.
The power of Ice Water will depend on the Magic of Marle and Frog. The higher the magic of both characters, the greater will be the damage caused by this Tech. The same goes for Glacial Freeze.
Ice Water may be a good alternative for Ice II and Water II, due to the high MP cost for both Techs.
Strategy
Ice Water is a very useful Tech in the Fiendlord's Keep, as it can affect all enemies in battle for a low MP cost, costing only 2 MP for Marle and Frog, plus most enemies of said place are vulnerable to magic (except for the Sorcerer, since this enemy absorbs all magical elements except Fire).
It can also be useful in the battle against Melphyx in the Sunken Desert, as Melphyx's defenses drop considerably when any Water Elemental Tech is used on it. It also allows the player to save MP and Ethers, due to its low MP cost. This causes the player to choose to use Ice Water, instead of Water II or Ice II, since both Techs require 8 MP to use.