"homeo" means "same"; in the case of homeostasis, this means "keep internal condition stable". The human body has a certain content in salt. Introduce distilled water inside and, through osomosis, the cells will start taking a lot of water in as they will try to be as salty as the environment, and that can dangerously disrupt them. An UV has about the same salt content as blood, so that the salt balance will not be upset.