First instance of the use of immersion in a breathable liquid for high gee flight?

This answer to I saw this article online that mentioned animal testing for immersion in water for applications in astronauts. What plans might require this? in Space Exploration SE includes:

I don't know of any planned missions requiring these high G's, but lunar surface launch with a linear accelerator or an Oberth maneuver around a neutron star come to mind. I believe the strategy was used as a plot device in Robert L. Forward's "Dragon's Egg" science fiction novel for orbital insertion around a neutron star.

Information on high g-force tolerance: What limits are put on mission profiles by maximum tolerated G-force?

The physics justification is that a biological body on a modest gravity planet is delicate against very high accelerations - it gets squished against the supporting surface (e.g. chair) and low density spaces like lungs get compressed to zero volume and so breathing is impossible.

By immersing the body in a fluid with the same average density as the body and making it also breathable, the body can survive with minimal damage during accelerations equivalent to many tens to a hundred times the equivalent acceleration of its home planet's gravity.

Question: What was the first instance of the use of immersion in a breathable liquid for high gee flight?

note: I'm particularly interested in spaceflight, but based on comments, answers related to atmospheric flight are also noteworthy.

The 1969 story Womb to Tomb by Joseph Wesley describes this sort of acceleration tank, though it is not totally clear whether the subject breathes the fluid or not. It is implied that he did because the protagonist Jim Grimes says:

The whole thing goes back to the twentieth century, when some clever idiot decided that animals ought to be able to breathe under water as well as in air if the pressure was great enough to raise the level of dissolved oxygen to a high enough percentage to support life. It turned out that he was almost right. The subjects died, but not because they couldn’t get enough oxygen. It was because they couldn’t get rid of enough carbon dioxide.

then a few paragraphs later:

I gestured toward the tank. "And while they were dumping me into the tub of salt water you see there, they were pumping my lungs full of an equally salty liquid—odorless, but of a very low viscosity. At least, all of the authorities claim that it’s odorless, but most pilots agree with me that it smells a little like the inside of a used canvas shoe. This liquid, at any rate, solves the problem of carbon dioxide poisoning, since it has more than double the dissolving capacity for the stuff than water."

The story doesn't actually say Jim was breathing the liquid, but he was presumably exhaling it for it carry away the carbon dioxide.

The earliest example I can find, since I couldn't turn up the occurrence bob1 mentions in Triplanetary is in the Joe Haldeman short story "We Are Very Happy Here" (1973), published in Analog, November 1973. This short was incorporated into The Forever War (1974) so it shows up almost identically there.

"I wasn't thinking it through, either. The way we normally use the shells is out of the question for Marygay, of course." I didn't like to think about it. Takes a lot of hypno-conditioning to lie there and have oxygenated fluorocarbon forced into every natural body orifice and one artificial one. I fingered the valve fitting embedded above my hipbone.

The "shells" are personal acceleration support devices that, after replacing all air in a body with "oxygenated fluorocarbon," pressurize to about 500 atmospheres (equivalent to an ocean depth of 5km, or 2.7 nautical miles as noted in the text).

E.E. Smith used a fluid tank for in-atmosphere acceleration in Triplanetary, first published in 1948. The following quote is copied from the 1975 edition, available on Project Gutenberg, and is also found in the 1948 novelized version (based on page scans at Internet Archive), though the initial serialized version (Amazing Stories, January - April 1934) did not include it, nor any of the background for his later Lensman series provided in the novelized versions.

The ship was a tremendous flying wing. A standard commercial job. Empty—passengers, even crewmen, were never subjected to the brutal accelerations regularly used by unmanned carriers. Phryges scanned the panel. Tiny motors were pulling tapes through the controllers. Every light showed green. Everything was set. Donning a water-proof coverall, he slid through a flexible valve into his acceleration-tank and waited.

A siren yelled briefly. Black night turned blinding white as the harnessed energies of the atom were released. For five and six-tenths seconds the sharp, hard, beryllium-bronze leading edge of the back-sweeping V sliced its way through ever-thinning air.

The vessel seemed to pause momentarily; paused and bucked viciously. She shuddered and shivered, tried to tear herself into shreds and chunks; but Phryges in his tank was unconcerned. Earlier, weaker ships went to pieces against the solid-seeming wall of atmospheric incompressibility at the velocity of sound; but this one was built solidly enough, and powered to hit that wall hard enough, to go through unharmed.

There's nothing in the text to indicate that it was a breathable fluid, so doesn't quite fit the exact question.