I must have a lot of endurance. I read this whole thread!
I'm not a contender at either running or swimming, although I enjoy both and would not want to give up either one.
My main sport is running, but I am injury-prone. If I didn't have the swimming, I'd be much more so. And when injuries keep me from running, I'm especially glad that I'm not without a means to stay fit.
Interestingly, when I first began masters swimming, I noticed improvements in my running times--also in my swimming, but that was to be expected. When I'm just running and not swimming at all, I can to some degree improve my running--but as a recent experience showed me, I do so at some cost. While training for a marathon and increasing mileage, I developed plantar fasciitis and had to cut back drastically on running.
So it was back to the pool for more swimming. And as a result I finally (don't laugh!) broke 2 mins. in the 100 freestyle). But I didn't do that well with an open water swim of 2 miles. (And I noticed that upper arm and shoulder muscles suffered in my training for that swim--but not to the degree that I needed to stop.)
Since I happen to love participating in both sports, I am less concerned about whether one helps the other as about finding the right mix of both.