Filling someone's chest and sinus cavities with fluid won't do much to strengthen the heart's ability to pump blood against G forces nor will it make blood vessels any stronger. Animals like fish and amphibians who live their whole life in the water may have an advantage simply because their bodies may be more fully filled with incompressible fluid.

One of the problem with pulling high negative Gs (say in a fighter or an airshow plane) is that the blood vessels in the pilot's eyes can easily rupture, causing a redout. This happens despite the eyes being full of fluid. A similar thing would likely happen inside someone's brain during high transverse Gs - the blood would tend to flow towards the rear of the brain, perhaps starving the frontal lobes while possible rupturing blood vessels in the rear.

Now, if you want to try it, knock yourself out.