And putting someone in fluid would only help a little – it would distribute the force somewhat over their skin, but that skin would still be pushed into their internal organs and whatnots.
Think about being in a submarine- let go of a wrench and it will still smash into the deck.
If the acceleration was only for a short period, you could use a big vat of fluid (slightly less dense than the body) as a shock absorber, letting them drift backwards through the fluid so they don’t accelerate quite as fast as the vat itself does, (as long as they don’t reach the wall before the acceleration stops), but metal springs on their couch would do the same thing.