I think you should concentrate on low contact stomach flattening exercises.
Swimming is good, it's easy on the joints, and focuses on the core - a good workout for your abs and back muscles.
Crunches are a definite no no. But an excellent option, instead of pivoting the top half of your body, is lifting and lowering the bottom half ... leg raises:
Lie on a flat surface, start with the knees bent and raise to your chest, as you get stronger you can do the exercise with straight legs for greater effect.
The great thing about strengthening your stomach muscles is that it will also strengthen your lower back at the same time.
For more info check out Mike Geary's guide to losing stubborn stomach fat and getting a flat stomach - truth about abs and flat stomachs
It's the top abs program on the internet.
Hope that helps.