One thing that seems to suffer with relaxed trail running is stride length. I think the pool is a great place to work on a long stride motion. At least there are no rocks or roots to trip over. The coolness of the water will keep your heart rate down, so it wont feel like you're working hard at all. When you get out of the water and feel normal gravity again, you'll know that it wasn't all play.
Some people find pool running very boring.
After a hectic day at work, I used it as a way to mentally relax, turning the simplicity into a useful feature. I had a knee problem in December. Ok, so it wasn't a great time or a PR, but I did finish. Course when I got home, the my orthopedic surgeon gave me the results of the MRI and that I had to have it scoped. But I did finish and I attribute it only to the pool running.