Thanks, great insight!

Interesting. Liquid ventilators do use pumps; I guess because, as you say, we can’t push the liquid fast enough with our own force. But I think some research setups only fill the lungs and then use a regular oxygen ventilator, so maybe it’s not that infeasible to survive in a perfluorodecalin-filled tank for at least a few minutes, before becoming exhausted?

I’d say perfluorocarbons, like perfluorodecalin. Harmless and clear, but they have huge oxygen absorption capacity, so you’d surely be able to breathe even if you sank your face in it (probably not fun to do so tho).