Fitness gain-water running vs road running?

I personally do not think so. I think water running is a great alternative when used while injured. I think is helps maintain some sort of fitness when unable to run, but I don't think it will increase your run fitness.

It would make you a pretty good water runner, but I don't think you can use it as a substitute to running and hope to reach your potential as a runner on land.

I did water running the first season I was injured and couldn't run for over 30 minutes at a time without a lot of pain. What I found when I did my big race is that the water running didn't hurt but it wasn't clear it helped. So the second season I didn't bother (used that time to do more biking which is my weakest sport anyway) and I had the same result* as when I did the water running.

I've read that water running can maintain run fitness but can't improve it and my own personal experience matched that.

(*In both cases, I found my legs giving out when I hit mileage similar to my longest run on land and I had to gut through any miles after that.)

If you are injured I think it is a great idea.  If you are not injured, then I think it is not a great idea.

First, water running is boring, as in, 'please stick a needle in my eye to make this interesting' boring.  (I've done 2+ hour 'runs' in water.  I know whereof I speak.)

Second, and more important, you do not get the same impact forces, and those are important to overall run fitness for a variety fo reasons.  (And for the same reason, you have to be careful about transitioning back to the road.)

I was able to maintain pretty decent fitness when I was injured, but I would never do it simply as a substitute for running.