Hold for 1 second, then repeat. Swim on dry land. This is great for extending and strengthening your lower back. Lie on your stomach and raise your left arm and your right leg. Hold for 1 second, then alternate with your left leg and right arm as if you were swimming.
Swimming is great exercise for the back.
A good exercise for acute low back pain is to get into a warm pool and swim. Know your limit. If the exercise you're doing hurts or aggravates your condition, stop immediately. Make sure that before you stretch any muscles it must absolutely be relaxed. It must be relaxed in order for it to stretch at all.