ESA (European Space Agency) have been studying this.
The article is so short, I'm going to copy it here. Link to source at the bottom
TLDR: 24G with air breathing, but suspended in liquid. 100+G with liquid breathing as well.
Is there an example design in nature for the perfect acceleration shield? In effect, there is, and it is the egg. We studied the coupling of water immersion with liquid breathing as a possible approach for the “perfect G suit”.
By completely immerging a man in a physiological water solution within a non expandable, rigid container, the increased fluid pressure developed within the cardiovascular system during acceleration is approximately balanced or even cancelled out by the gradient of pressure developed in the liquid tank outside the body. At the same time, water immersion increases tolerance to acceleration as the acceleration forces are equally distributed over the surface of the submerged body. This abruptly reduces the magnitude of localised forces and a homogenous hydrostatic response of the whole body is induced, with evident benefits for blood and lymphatic circulation. The limiting factor is the presence of air in the lungs. Once under acceleration, the immersed subject experiences an augment on external pressure, which will casue squeezing effects on his chest, until all the air present in his lungs is removed. This fact limits the applicability of the technique to a sustainable acceleration of 24 G.
In order to overcome the limit and reach the real potentials hided in water immersion, it is possible to fill the user’s lungs with a fluid. In this way there won’t be squeezing effects. The problem, then, is: how is it possible to breath with liquid filled lungs? The answer came from the field of clinical lung therapy. Here, the use of perfluorocarbon for liquid ventilation was longer studied, demonstrating the feasability and safeness of the concept.
It is hard estimating an ultimate acceleration limit possible with this set-up, but it presumably can be higher than hundreds of G. The ACT is working to assess the application of liquid ventilation for water immersed astronauts, in order to identify the space requirements and to address future studies, designed to overcome current limits of the technique.
[The article cites some more detailed papers]