I used to do some of my hardest workouts in the water. As I recall, they were basically unscripted -- mainly lots of hard intervals, like up the pool and back, with fairly short recovery jogs. It was all about the effort. I loved that I could push myself to exhaustion without any big concerns about injury. I did these sessions when I was healthy, not just when I was injured and unable to run on land. I felt that they were very productive, and I usually felt very good after doing them.
In the early years, I used a water-ski jacket; there were no "aqua jogging" belts on the market back then. In later years, I used no flotation device. I could do hard workouts either way; the difference was primarily in my "running" form, and perhaps also my stride frequency, which might go down to about 150 (or slightly less) per minute when I didn't use any flotation device.
In addition to intense workouts, I did some long sessions, up to something like four hours of continuous running without a flotation device, but you better have a very good reason for doing something like that. (In my case, I was trying to train through a stress fracture during the months leading up to the Olympic trials, and I just wanted to make sure that I did whatever I could.)