Shameless plug but I’ve built a bunch of water cooled computers and even the more reliable ones like the Swiftech build below with John Guest push fits (rated for 25 years use) do eventually leak.
https://www.leonroy.com/oldsite/img/IMG_1802%20(Large).JPG
Regarding distilled water, it is indeed not very conductive at all but it is also quite reactive and if it touches electrical traces and PCBs starts to leach ions from the surrounding until it starts to conduct at which point the machine will short circuit (speaking from first hand experience alas).