When it comes to water, there are a few things to consider when determining if it needs a hechsher. The first is whether the water is fit for human consumption. If the answer is yes, then the next question is whether the water has been purified.
Water that has been purified does not need a hechsher as long as it meets certain criteria. The water must be from a natural source, such as a spring or well. It must also be free of contaminants and impurities. Finally, the purification process must be done in accordance with Jewish law.
If you're unsure whether or not your water meets these criteria, it's always best to check with a rabbi or other knowledgeable individual before consuming it.