Exercise both legs. Do in sets of 30 to 50 repetitions three to five times per week. As strengthening occurs, increase size of tubing or distance from tubing. The medial rotation of the leg causes the feet to rotate inward. At this orientation, the foot surfaces no longer effectively engage the water.
More adjustments are necessary.
The lower leg must be stretched to allow the foot to rotate out farther. Then it works. To stretch, press the right elbow forward and pull the right hand back. This causes the foot to rotate outward and stretches the structures in the lower leg. This is an extremely powerful stretch.