Take two dumbbells and stand in the shallow-end. Bend one leg in front to let your shoulders in the water. Keep your other leg stretched behind. Hold the dumbbells with your arms slightly bent in front, at your torso level and shoulder-width apart. Let your palms face inwards.
Push your arms in and out fast in small range of motion as you go down and up. Lean your torso forward as you push your arms down. Lean backward as you push them up. Make sure to move your torso and your arms as one unit. Do not spread your arms beyond the width of your shoulders and focus on moving your arms fast. The speed of your arm movement aims to create tougher resistance.
Push your arms twice as you inhale and twice as you exhale. Provided that your arms are moving as fast as shown in the above gif. Continuous breathing is very important when performing fast movements.
Focus on using your inner chest muscles with each push in. You can also feel your shoulder muscles by moving your arms out. If you feel that your muscles are not working sufficiently, try to speed up and push more water along.