I just got back from a workshop in PA with Mary Saunders that incorporated some pilates, tai chi, and yoga work in the pool and found this variation on the bicycle to be very effective for engaging the core.
Using a noodle or a belt: legs are doing a bicycle movement. Bring this movement to within 2 inches of the surface (don't break the surface) and hold. Then take the move down in the center and then diagonally to either side. Finally, bike up and down a "hill" without breaking the surface of the water. The "hill" is merely a circle () off to the side going up one side and down the other side--all while biking.