What is the best water used for cooling in cars

Is your real question about water, or benzene?

I'm guessing you mean adding benzene to your diesel not your cooling system. I wouldn't add it myself. There's plenty safer and better ways to add power. I'm more concerned about the lubricity of the new diesel fuels out there myself and would be more inclined to add a conditioner or a bit of oil to your fuel instead.
For your cooling system it really depends on your climate and type of diesel as to what coolant to put in. I do avoid any dex-cool coolants like the plague as I've seen first hand what they've done to some diesels that I've used. I