swimming with a pull buoy (thing that you put between your leg/knees, gives buoyancy and swimming without kicking easier)? Not sure how this would be for the hip flexor aspect of your injury, but I swam without kicking this way for about a year due to knee injuries and it permitted exercise wo knee strain for me (during a time when I couldn't ride and even walking was difficult). Good luck with this. It;s really really hard to adjust to limited mobility/activity levels when you are used to getting a good amount of exercise.