How would being immersed in fluid help someone withstand 200 Gs?
Sure, having your lungs full of incompressible fluid would help, but I don't think it would do anything to help your heart pump blood. When a pilot pulls high positive Gs, the heart's inability to pump blood to the brain is what causes grayouts and blackouts. High negative Gs causes blood to flow to the brain and can cause redouts. Even with the transverse Gs (front to back) like you'd pull if sitting perpendicular to the acceleration vector, if the loads are high enough, it seems likely that blood would tend to flow away from the front of the brain towards the back. Do that long enough and you're likely to face brain damage from oxygen starvation.