Hmm... How about swimming with a kick board??? I am not sure how serious your stress reaction is. The first time I had a stress fracture in the femur the doctor had me on crutches for 6 weeks. So walking at that point was obviously bad. The second stress fracture was further down the femur, and less critical. The doctor told me not to bother with crutches. I think it took me a lot longer to heal. I ended up starting to run again, and re-injured it. If I had gotten on the crutches, it may have healed faster, I don't know.
So... like you said, walking does put impact on the femur. If the muscle contraction from water running is irritating the femur, then I would think walking would be worse.
I'm a mathematician, not a kinesiologist, so I'm not really trained in biomechanics (or spelling) but I would be careful if I were you.
I'd say, try walking 5-10 minutes or so first if you haven't already been doing it. If no problems, try 15-20 minutes. If you can, try to do it on some grass or other soft surface. Avoid sidewalks and roads. Don't just go out and do an hour without building up to it. And I would build up gradually. I don't know, maybe 5 mins per walk per week.
Everyday may be too much also. Maybe mix it in with some kickboard swimming at the pool.
Good luck with the injury.
For what it's worth, I can tell you about my first experience with a femoral stress fracture. Maybe it will help.
I had 6 weeks on crutches. Took about 2 weeks doing nothing (I was depressed and pissed off about being injured, and the doc told me to pretty much lay off everything). Then, I would go to the pool, started off doing 30 mins, worked up to an hour, and even 75 mins. I was in the pool 6 days a week, and did some upper body and core weights and stuff maybe 3 days a week. I'd alternate between swimming and water running. After about 2 weeks of just easy water running I started doing "workouts" in the pool, basically 1-3 min pickups with 30secs - minute recovery.
After about 6 weeks, I started to do some biking to break up the pool. Started with easy biking, 30 minutes or so. After a week, did some intervals on the bike, kept the session to about 30 minutes, maybe up to 3 min pickups. All seated biking on the indoor bike. Kept the resistence light, and the cadence high. gradually built it up, and then switched to some outdoor biking and stuff like that. Also eventually got onto the elliptical, but by then it was nice enough to bike outside, so I prefered to do that.
After about 4 weeks of biking, I started with some running on grass (this is about 10 weeks since the injury.) I followed the advice of a friend who coached D1 and he himself had multiple femoral stress fractures as an athlete. I kept up with the cross training, pretty intense by this point, and started with 3-5 mins of jogging on grass, gradually building up over the next 6 weeks to 40 minutes or so of jogging on the grass, 3-4 times a week.
It was a slow build up, but the cross training will keep you fit. I was originally diagnosed with the stress fracture in February. By July I was doing 10 mile runs. By October, I ran a PR in 8k cross country. By November I ran a decent marathon. I kept it up over the winter, and I thought I was being smart. I ended up setting a huge PR in half marathon in March, and followed that 1 week by another femoral stress fracture, same leg, lower down the shaft of the femur.
Hope your luck is better than mine!