One of the reasons you use deionized water in watercooling systems is because it doesn't conduct. I did once spill coolant on my motherboard when I had my old (and more messy) setup, nothing happened
Just make sure that you dust your case out frequently, because if it does leak the deionized water will become conductive again if it mixes with enough dust.
If it did leak and fried your stuff, then the fact they have filled it with "unsafe" water would be enough to have a case against them. I doubt they would be dumb enough to do that though