STARTING POSITION:
Dive into a pool. The water should be at the height of your neck. Stand straight with your feet shoulder width apart. Your hands should be in a neutral position.
MOVEMENT:
Raise your right arm in front of you and fully extend your left shoulder backwards. Then fully extend your right shoulder and raise your left arm in front of you. Repeat the movement until all repetitions are completed.
TIPS: While your are doing the exercise, move only the arms, make sure you don't move any other part of your body. The exercise key is quality over quantity. The trick is to perform the exercise using proper form.