Hold two dumbbells and stand in the shallow-end with your feet hip-width apart. Hold the dumbbells in front of you at your lower abdominal level and let your palms face downward. Keep the dumbbells hip-width apart and slightly bend your arms.
Note: any object like a noodle works too if you don't have aqua dumbbells.
With the dumbbells still in front of your body, jump and raise your knee high reaching and trying to touch the dumbbell. Reach with one leg at a time. Start slow as shown in the first gif. Speed up as shown in the second gif and feel your heart rate going up.
Note: Always keep your back straight and gradually increase the speed.
Inhale as you reach with one leg then exhale as you reach the other. As you speed up, your breathing should change accordingly.
Feel your lower abs as you raise your knees. Feel your quadriceps as you push your legs. The faster you go the more resistance you create for those muscles.
This is a great exercise for people who aim for good looking abs and burning some calories.