A non-conductive fluid is called a dielectric, examples of these are R134a - the refrigerant used in your car or 3M sells a line, one of the fluids is called FC72 I think. There are NO water based fluids that are dielectrics, there is NO additive you can put into water to make it non conductive.
Some of the issues with using a water based system are freezing & organic growth, thats why they make antifreeze. You could build a system using this but it makes the water less thermally conductive. It just depends on your total heat loads you are trying to disapate & your expected ambient temperatures. You need to size the radiator to match these, bigger and you get closer to ambient with smaller fans, smaller & you need more air flow. You also have to be careful about dewpoint. If it's really cold in the trunk and you pipe this very cold fluid into a warm & more humid environment you get condensation - put some ice in a glass of water, that's what happens.
Aside from the CPU/GPU you may need to cool your inverter as well, at least have it in a spot with good airflow.
There are a couple of startups developing R134a based computer cooling systems you can learn more from their sites, try tfandf.com