Personally, if I'm staying in the pool for an hour and doing fairly brisk front crawl for the whole time, I'd want to give my body at least four hours to recover from the run and workout (I'm presuming you mean a gym-based workout). On days when I do this, I use it as a replacement for running. A 15 minute splash around, on the other hand, is great after a run or especially workout to loosen up and cool down.
I tend to try to seperate resistance-based workouts and running by at least four hours too. The exception to this is if I'm using a slow 3 miler as a warm-up for my gym work. I'm a firm believer that the body doesn't take as much from over-training.
Of course, triathletes will have a different training schedule.