The difference between water and air is pretty much only the density. Other than that, they act very similar.

This being said, to do a test on an airfoil using water, you can use a lot lower velocities of water because the density is a lot higher. This results in the same reynolds number.

Yissht, you may have done the experiment, but it is likely not valid because you did not take into account that the water has a much higher density than air. This will drastically effect the reynolds number and how the airfoil reacts with the environment. To figure out what velocity the water should be at to compare to air, you need to know the velocity you want the airfoil to travel at through the air. Then calculate the reynolds number over the airfoil. Then use that calculated reynolds number to calculate the velocity of water that is needed to obtain the same reynolds number.

Then you need to regulate this velocity and put it over your airfoil and see how it reacts. This will be a valid test to see how your airfoil performs in air using water.

This is a very good method to use when evaluating high speed planes because it is really hard for the average joe to make a wind tunnel that will produce very high speed air. But if you use water, it can move at a much slower speed that is much easier to obtain.