Use distilled (pure) water. Contrary to popular belief, water doesn't actually conduct electricity. It's just the dissolved salts in the water which allows it to conduct electricity (without going into too much chemistry). Keeping it pure, or near pure, is hard, but that's very important in water cooling. If you want the system to look funky then use coloured tubing instead of dye.
Antifreeze is also an option, but it has limited use in a pc cooling loop. The antifreeze prevents water from boiling or freezing, but since pc operating temps typically don't experience the temps to make it do either, AF is basically optional. AF also prevents corrosion, but that depends on the metal of your cooling block, but with aluminium blocks that isn't really an issue. I would do some research before using AF.
Also, AF contains alcohol in it, mainly ethanol and some other stuff which i don't know. Since ethanol heats up much easier compared to water, it will absorb less heat per degrees celsius, meaning it won't be that effective compared to straight water.
It's also generally agreed to use some sort of biocide in your mixture since you can have some nasty stuff such as algae or whatever growing in your tubes.
Sorry for the info dump and semi chemistry lesson :P But better you know now than after you've had a stuff up.