Is it true that under water human body can sustain higher levels of acceleration? In several sci-fi novels, I've read about humans being submerged in water (or water-like substance) to allow for a higher acceleration. Now, I'm really curious. Is this true? Can you accelerate faster without sustaining any sort of harm if you're submerged? 
Is there any formula which would give me a relation between effects on human body of acceleration when not submerged vs effects when submerged in fluid with different levels of viscosity? 
 A: Yes, immersion in a fluid can assist in handling G-load.
I assume that the high G-loads in sci-fi novels are in stories about fighter spacecrafts capable of short bursts of very high acceleration.
The key factors are the incompressibility of fluids, and buoyancy.
There are sharks that roam between depths of kilometers and right at the surface. At every depth their buoyancy is the same. A high G-load would increase the pressure of the water, but these animals are not affected by those differences of pressure.
The buoyancy outside and inside the body distributes the load evenly, so that there will be very little tendency for one part of the body to move relative to other parts. In particular, the blood will remain distributed evenly.
I guess that if on some other planet an intelligent species of shark evolves to a point where they are ready to do battle in space then they are ready to go for extreme G-loads. 
Can that be recreated for humans?
Of course you have to replace breathing air with a fluid; air filled lungs collapse when the pressure increases. There have in fact been experiments where humans would have their lungs filled with a fluid that has a high capacity for dissolving oxygen in it, but the molecules of the fluid itself would not pass into the bloodstream. 
The amount of oxygen that will dissolve in water is not enough to meet the oxygen requirements of the human body, you have to use some other fluid. Secondly, for fluid-breathing special pumps must work the fluid in and out of the lungs, because the lung muscles are not strong enough to move that amount of fluid.
But getting the lungs sorted is not enough, I think.
There are a number of gas filled pockets in the human head: the sinuses, the inner ears (and there may be other ones that I don't know about.) When a diver descends he has to equalize pressure in those pockets to the outside pressure from time to time, and the same on the way up.
It may be that those gas filled pockets in the head will be limiting factor on dealing with sudden change of pressure.
Also there is the matter of the sensory organ in our inner ear that feels how you are oriented. It may be that under extreme G-load those sensory organs will be touched so hard that there is acute pain.
A: There may be some truth behind this statement, as the force can be distributed better over the surface of the body in a liquid ( http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/dropping-eggs/ ). I guess the effect would be even greater if the acceleration were due to gravity so the force would be distributed over the volume of the body, so relative acceleration of parts of the body would be small.
A: If accelerating at $7G$ while submersed in water, breathing air from a regulator (scuba regulator would do), the effects on the body will be just like being subjected to $7$ bar pressure here on Earth, at $60$ meters depth. 
You will be narced and saturated just like a scuba diver, but otherwise you would feel perfectly fine. 
Breathing other gases, like heliox, you could be subjected to up to 50G, accelerating at $500 m/s^2$ and  decompression would be needed, like slowing down the acceleration over a longer time.
A: If a human body is submerged under 1 meter of water and the acceleration is 5 G, the water pressure would be as if the body is submerged under 5 meters of water. However, if the body is submerged under 0.1 meter, at 5 G it would feel like .5 meter. This way, it should be easy to get to 100G under 5 cm of water (which would correspond to 5 meters of water pressure). This would be very easy to sustain of a long time, without using liquid filling lungs.
When sending people into space using a system like Star Tram, I would give the astronauts (special) scuba gear, put them in a shallow tank and shoot them into orbit with 100 G (1000m/s^2) for 9 seconds.
