Strengthening core muscles will improve your posture and thinking about posture will improve core muscle tone! It's great how all that works together. What does water do? I'm so curious - is it all about lubricating the discs?
To avoide extra stress on your back when working the core muscles, consider using a large, inflatable stability ball for support. You still get an incredible workout with crunches, and you're not straining your back as much as you lift and lower your body during the exercise.
I saw this on sale today at Amazon.com. It was a desk chair, but instead of a regular flat seat, there was a balance ball like the stability ball used at the gym. It was certainly interesting looking. It was described as designed to help build a healthier back and relieve pain. Perhaps something for people like us to research and possibly invest in.