You can use it to enchant certain types of equipment. I'd check out ILK's Equipment Enchantment guide in the Strategy section for more details. I think it may be in AT already, but I'll bump it for refernece.
Aqua only enchants certain equipment, you generally need it on later levels of enchanting. The first four levels to my knowledge only need inferno. After that is when they need aqua. Hope this helps! Stew