Yes, air compresses and water does not. However, without a wind tunnel, water is one of the best methods to use. Many auto manufacturers use water turbulence testing as well. I was not nearly as concerned with localized compression as much as flow characteristics over the body. My main point of interest was trying to get 40% of the air flowing over the body and 60% flowing around the body for downforce reasons. I got it really close with the water testing. I was only off a tiny bit in my nose downforce calculations using the water flow method. So, I merely added a chin spoiler to increase the nose downforce just a touch and bingo, it ran very stable.
Anyway, I also have a smoke pen and an annemometer. Putting the car in a gentle breeze and following the smoke line shows the same flow characteristics as my water test showed.
There is still so much guesswork in this, though. Sometimes it is best to just go out and run it to see how it works!
Matt