It depends on which type of engine, how big and how much water it consumes per day. You have to strike a balance between cost of treated water and cost of maintenance & downtime due to use of cheaply available water. You can refer manufacturer's instructions too for help.
use distilled or de-mineralized water. It doesn't leave behind any mineral layer which gradually reduces heat transfer. In due course of time the engine will begin to get overheated and get damaged if you use ordinary water.
The one stated in the original equipment manufacturer's operation and maintenance manuals, of course.
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