tritium is even heavier water.

Technically, the “light water” is just an oxide ion: an oxygen atom with two extra electrons, filling its second electron energy level.

“A substitute for oxygen?” “New Oxygen Lite™!” — MST3K, “The Crawling Hand”

Edit: I would like to learn from the downvotes, but neither has been helpful enough to leave a comment.

Mmm. Delicious free hydrogen radicals. So refreshing. Not harmful at all.

But if you really want “light water”, may I suggest replacing all 10 necessary protons with anti-muons? It won’t last long*, but it would reduce the theoretical mass of the molecule down to less than 1/8th**

And the resulting nuclear explosion would be a great topic for an xkcd What-If.

*: Muonium is a thing. It’s “hydrogen” made from an anti-muon nucleus and an electron orbiting it. It decays as the muon does unless there’s some kind of matter-antimatter annihilation in which case it’s a bit more vigorous. I am not sure whether you can put anti-muons in a nucleus along with regular neutrons though. No harm in trying, right?

**: Excluding quantum weirdness affecting the mass in other ways.