Your arms are not meant to propel your body. We, all human beings, are not built that way.
Your core muscles — lower stomach and lower back muscles, butt, and upper thighs — are designed to propel your body, so engage your core. Swim using the same large muscles you walk and run with or, put another way, swim from your core outward, from the center of your body outward.
Chimpanzees, gorillas, and other primates, on the other hand, do have arms designed to propel their bodies. They walk on all four limbs. Their strength is in their upper body.
Humans walk upright. Your strength is in the center of your body.
