Use an anti-corrosion additive, pure water will rust inside the engine the same as tap water in the presence of oxygen.
The Thor engines use an OAT 3-year coolant, I've been impressed with the way that keeps the cooling side clean on my Discovery 2, but it is used in 50/50 concentration and isn't cheap, but good things rarely are
ethylene glycol antifreeze as was common is cheap and perfectly good to use in 3.5 v8 ,all those used without coolant (antifreeze)ive stripped have had signs of corrosion and electrolysis but not with proper coolant