An underwater treadmill sounds fascinating. Good luck with the injury - hang in there.
What a great way to beat the doldrums with a nice swim

If you can do 2.5, you can probably stretch it to 3.1, what's another 900 meters or so between friends. I know when my hips are tight I am a crappy swimmer, but when I had a hip injury I was still able to swim pretty well - especially after PT before I could run anything more than "a block" or whatever.
Is it in a lake, or river, or something else?