Both. It works well with distilled water. I was never able to see any inprovement when mixed with coolant.
It does improve heat transfer, is a very good corrosion inhibiter and pump seal lubricant. It offeres NO freeze protection. It also requires annual replacement.
Tho I am sure I would get better results with just water and Wetter only, I did see some improvement in the car and much improvement in a boat closed cooling system, both with about 40% antifreeze.
There is also this for even better results;
http://www.engineice.com/
Damn! Have you tried this on your car or any engine?
Very similar to the product Prestone sold for a few years as a non-toxic antifreeze.
Its also available in the drug store in powder form, sold as Mirralax......exact same chemical in medical grade form.