https://en.wikipedia.org/wiki/Blood_substitute

In the 80s I had a non-biologist friend who told me about something in development which could act as a blood replacement. They wanted to fill a swimming pool with it, sit on the bottom of the pool, and meditate. I suggested the viscosity of a pool filled with anything like blood would be a whole lot more viscous than air, and so would require a lot more muscular effort to move in and out of their lungs. And so the experience wouldn't be as relaxing as they thought. All these decades later, and researchers are still working on it.