Cell of highly refined water, made from placing a Water Cell into an extractor to make a hydration cell.
Simply right-clicking the cell onto some IC Crops will consume some of the water and hydrate the plant for a while. Hydration Cells can no longer be used inside a Crop-Matron. They now only accept Universal Fluid Cells that are filled with water, or water can be pumped in directly.
See: Crafting Guide
IC2 v1.103 and earlier:
As well, we recently started experiments with a liquid called H2OEX2, simply referred to as "HowToHex". HowToHex can be refined from basic water by using the patended Extraction device "Dejuicer Device of Doom", created and sold by our co-concern "Industrial Craft Machine Corp.". Fill a metric litre of water (simplifie to the intelligence level of a common employee: a bucket) into a patented "Tin Cell" and refine this cell TWICE within the Extractor. Twice, not once, not thrice. Don't even count to one extraction, unless you intend to proceed to count until two. The four is entirely irrelevant.