Water is to a fish as ?? is to a human.
A fish in the ocean experience is water as it's normal every day environment in fact it does not even know what water is.
The analogy here for us humans is what are we experiencing every day as our normal environment so much so that we do not even know what it is.
The simple short answer is consciousness. Every day with humans swim through consciousness without even realizing that it is there. Consciousness is our normal environment. Consciousness is our water.
To a fish almost everything is water. I think our equivalent is; to humans nearly everything is consciousness. We just have not recognized it as that.