Is gravitational force affected by intervening medium? If we leave a iron ball and feather into the water, feather returns to the surface and floats or moves into the water slowly. On the other hand, iron ball (of certain mass greater than mass of feather) moves into the water directly. Here, both iron ball and feather are accelerated towards the earth to the same extent. Then why does feather moves slowly into the water than iron ball? Shouldn't they reach the earth at the same time.    
If we assume that water medium is making the feather to accelerate to lesser extent than iron ball, then doesn't it mean gravitational force is affected by intervening medium?  
Similarly if we consider a feather and iron ball to be left from certain height, I hope iron ball reaches the earth first and then feather. The reason I think is because of air resistance, doesn't this mean air medium or intervening medium is affecting gravitational force?   
We know that gravitational force is not affected by intervening medium, but it is contrary to the above situations. Is it that gravitational force is affected by intervening medium? 
I might have misunderstood sometimes, if so, pardon me and explain.
 A: Your thought experiment of dropping an iron ball and a feather need not be in water; in fact, it is more commonly considered in air, but the pertinent facts are the same.
All objects, regardless of their mass or composition, are accelerated identically by gravity. 
But within a particular medium, the acceleration of particular objects might be impeded by greater resistance than that of other objects. In  ordinary air, your feather will fall more slowly than an iron ball, because of air resistance. 
A: As you said, if feather has a stronger decelaration, is due to air friction (not because iron is more dense than feather or something).
The same should hold in absence of gravity: if you give a boost to these balls, then the feather one will stop first.
From this last consideration, I'd say that this medium does not affect gravitational force, but just it adds its effect to the gravitational force one.
A: To analyze whats going on you need to sum all forces that affect the objects in question. For each object, the feather and the ball, the forces of gravity are identical. The other forces are not.
A: There are multiple forces involved in your thought experiment.  Namely, the gravitational force and the 'force' of air resistance.  This does not mean that the presence of the air affects the gravitational force.  
As another thought experiment, consider two planets orbiting in space.  Somewhere between these two planets there is a point where the gravitational force goes to zero (side note: there are actually 5 of these points in a co-rotating frame, known as Lagrange points).  At this point the net force on an iron ball or a feather is zero even though the gravitational force from both planets is still acting in full.
A: Even though the people who replied are correct, fun fact: the speed that gravity is propagating is very slightly affected by mediums. Here are 2 pages from Kip Thorne's lectures at Les Houches. Gravitational waves are affected by a medium through a dispersion relation, but the effect is ridiculously small and is neglected. 
A: Let's use a spring scale in water s.t the spring scale is sealed in a air tight glass box containing only vacuum so as to remove the buoyancy. Putting both the feather and the iron ball of same masses on the scale we notice that the spring shows the same reading(say $a_0$) for both feather and iron ball. Hence the gravitational force on both the feather and iron are same.
Now let's repeat this experiment by putting the spring scale enclosed in the box in the air(or oil or some other material). We notice that the spring scale again gives the reading $a_0$ for both feather and iron ball.
Hence we conclude that the medium does not affect gravitational force.
