I think we need to make a small experiment to figure this out. I don't think it matters wether it's in space ore here on Earth.

Take e.g. a small plastic container and fill it with water so it's about the same density as water. Put it into a bucket of water. Rapidly push the bucket sideways to see if the bottle inside moves with the water or if it crashe sagainst the bucket wall.

Any volunteers?

If the container doesn't crash into the wall, the water provides no damping so the human would not benefit anything from being submerged.

If the container does crash into the wall the water would slow the human down but the human would hit the tank wall at some point if the acceleration lasts long enough.

Yup, basically. And when they are pressed against the wall then the person will be subject to the same G force as the tanks movement.

If there were small fluctuations then yes, the fluid would slow the humans movement compared to if they were in an air-filled vessel. If the G-forces were over a prolonged time, the the humans would at some point depending on the size of the tank reach the external wall of the tank and then the liquid wouldn't help any more.

It's like if you were in a car crash and in stead of airbags you had filled the car with water. It wouldn't help much because there is not much water around you. But if you had several meters of water between yourself and a fixed boundary then you the water would slow you down before you hit anything other than water.

In outer space there isn't any turbulence since it is a vacuum, so I can't see much use there though. Also a fluid filled container would be very heavy which is a big problem to launch up into space.