This is completely anecdotal of course but for the 8 years I've been running watercooled PCs (admittedly professionally built), I never had a leak. Or a pump fail for that matter, but even if one did fail, the CPU will shut off.
Furthermore, you are supposed to use distilled water or non-water based coolants with high electric resistance.
Furthermore, you are supposed to use distilled water or non-water based coolants with high electric resistance.