Dehumidifier water can be used for watering container plants and in hydroponic reservoirs. It reads 10ppm or less on a TDS meter. Water that sits stagnant can harbor mold and bacteria however, so you may want to oxygenate the water before using it. Oxygen levels are easily increased by shaking it in a jug or adding hydrogen peroxide. This will kill the harmful bacteria and your plants will benefit from the extra oxygen. The dehumidifier water is very soft water, since it has been recondensed from vapor which lost its mineral content when it evaporated. Many people use it to soften their tap water.
WARNING: Dehumidiers often employ copper tubing. If the resulting water is 10ppm its possibly 10ppm of copper. Adding this to your reservoir every time will start concentrating the metals-- since plants only need under 1ppm of copper, using this water could accumulate to the 100ppm toxicity level.