Repeat, over and over and over. Once the above exercises have helped you learn where your abdominal muscles are and how to control them, the next step is to turn the movement into a roll. Just keep doing the exercise, but think about doing it gracefully with an undulating spirit. Flow from one position to the other. Listening to slow, sinuous music will help a lot.
Different things work for different people.
If the above wasn't enough to get your stomach rolling, maybe you need to vary the technique or do the exercises in a different location from where you normally practice. Now try it. Then try to roll. The response of the water to your abdominal movements may help the movement come more naturally. Also, the warm bath water will encourage the muscles to relax when they need to.