Water comes naturally on planet Earth. Water is an important part of survival and is used for drinking, bathing and swimming.
Appearance & Side Effects
Unlike Dark Water which is red and has terrifying side effects, water is blue and is an essential part of living & doesn't do any physical harm to living things (humanoids, slugs, etc).
Purifying, Neutralizing & Corrupting
When it comes down to special properties, water doesn't have any, it's pretty much neutral, much like in the real world. But since it's neutral it can easily be changed.
Water can be turned into Dark Water by being ghouled by a Goon Doc or by simply ghouling the Water Elemental. Once the water is ghouled, slugs will naturally start leaving and the cavern(s) will start to die.
Water can be turned into Light Water ("Lightwell water") by being purifying by a Boon Doc/Energy Elemental (but so far this is only a theory). Once the water is purified, ghouls that are exposed to the "Light Water" for too long, will be turned into slugs.
Once water is changed, the only way to change it back is by neutralizing it using the opposite slug(s). Once the water is neutralized, things will start going back to neutral.