"Aqua jogging" works well to help injured runners maintain their fitness while not continuing to aggravate running-related injuries. No one is making claims (at least here) of anything other than that.
It worked for me. What you're saying is true about learning to do things correctly, like fixing running form a la POSE or Chi running or Pirie or whatever you like. I didn't know about these 10 years ago when I had trochanteric bursitis--all I knew was I had a triathlon coming up and I really sucked at distance running (yeah, I consider a 5k distance running).
There is benefit to it. You might not agree with the approach. Complete rest is an approach I don't always completely agree with--increasing bloodflow to an injured area is a good thing--if pain can be avoided while healing and maintaining fitness, why not?
I got my trochanteric bursitis from switching directions on a track mid-workout (silly me, trying to balance out the turns)...it wasn't necessarily because my form and technique sucked. Talked to my triathlon coach about what happened, learned my lesson not to do that again. So, after I learned my lesson, you're suggesting your approach to my problem would have been to completely rest (this took several months to completely go away) and lose the fitness I had gained previously? In my situation, I personally would not have gone for your advice.
Jonathan, your idea of a stationary boat while rowing the water sounds like a great way to splash around, but if it actually worked, I think rowers would have already figured it out.