Hold aqua dumbbells/pool noodles with two hands. Lay on your front with your arms straight in front and stretch your legs back. Keep your legs hip width apart and slightly below parallel. Put your face out of water
Bring the dumbbells/noodle and your knees closer together. Think of squeezing your abs. Keep your movement under the water. Then extend your arms and your legs together, back to the starting position.
Use your core muscles with each extension.
Inhale as you extend. Exhale as you bend
You can perform the same exercise with single noodle or any floating device. You may as well put on a floating belt if you are struggling with keeping your body floating in the deep-end. To increase resistance, you can put on foot floats for more resistance.