Stand in the shallow-end with your feet hip-width apart and your arms stretched out in front, at your shoulder level. Keep your back straight.
This is a combination of leg squats and heel raises. With straight back, bend your knees as you lower your body until you put your shoulders under the water. Stretch your legs and go all the way up, on your toes, as high as you can. Then down on your heels and repeat again. Perform continuously without pausing. Don't bounce off and keep your movement controlled.
Inhale as you go up and exhale as you go down.
Use your quadriceps to straighten your legs and use your calf muscles to raise your heels. Because you will be using your bodyweight as resistance in this exercise, you can create a tougher workout by keeping your arms above the water all the time. This will increase the weight on your legs.