Heavy water is not a radiological hazard, but it's toxic [1]. I doubt there have been definitive experiments on humans (for obvious reasons). I wouldn't try substituting it for ordinary water.
It is toxic when you ingest >20% of your body weight. Let's just say that that sort of thing won't happen by accident.
(And to point out the obvious, every other liquid apart from drinking water is more toxic when you ingest literal bucketfuls, including harmless household liquids like vinegar, shampoo, ethanol, olive oil.)
[1] https://en.wikipedia.org/wiki/Heavy_water#Effect_on_animals