This is the aqua version of the Plank Knee to Chest exercise. This type of exercises I prefer to be done in the water, where your body is safely floating and your legs kicking against water resistance, than on dry-land.
Planks are great for core muscles but they also work other muscle groups. Although performing it in the water is safer, you'd still want to keep a good posture during the exercise.
In the deep-end, put a noodle under your arms (or use any other floating device) and float upright. Rest your hands and your forearms on the noodle and relax your shoulders. Keep your legs straight and hip-width apart.
With a noodle under your arms, lay on your back in the deep-end. Rest your hands on the noodle and relax your shoulders. Keep your legs closed together, straight and slightly below parallel.
Aqua Exercises for Abs and Lower Back Muscles