50% mixture of engine coolant and water is used as engine coolant.
Tap water if you know its soft and passes drinking standard, Ideally you use reverse osmosis or distilled water to mix with 100% coolant to make a 50% strength mixture.
If you use plain water as coolant then you will end up with a corroded block and head, holed radiator and a huge massive mess of problems - which will further be messed with "hathora" mechanics using weed smoke brains who have no concept of how does the system exactly work.