How density of fluids is would affect inertia? I was watching that movie Event Horizon and they use water as an instrument to protect passengers of a space vessel from 30G acceleration, so my question is about a similar scenario, what would happen if you have a sealed container full of water in space and put and object inside that has the same density as water and then you move around the container, would the object move towards the sides of the container or it would remain still?
 A: Uniform acceleration. It's quite easy to tell you what happens in a sealed container with solid walls under constant acceleration, since this is equivalent to the situation where the sealed container is at rest (or moving with constant velocity - Galilean relativity) in a uniform gravitational field.
The body fully immersed in the fluid would float at rest w.r.t. the container, since its force of inertia (equivalent to its weight) would be equilibrated by the Archimedes' floating force due to linear varying pressure in the fluid, in the
opposite direction of the acceleration (due to Stevino's law).
Arbitrary motion. If you move the container around with an arbitrary motion, I don't think it's so easy to say what's happening inside the sealed container, and it's unlikely that the fluid stays at rest w.r.t. the container doing general motion, since the fluid can easily deform and move performing a non-rigid motion.
Anyway, with 30G acceleration, the pressure gradient in water could be so high that very high pressure can be reached in very small space, so that it could be unfeasible for a human body to bear that values of pressure.
