Swimming uses every major muscle in the body! Including, but not limited to, core abdominal, lower back, shoulder, forearm, upper back, glutes, hamstring and hip flexor muscles. Who says swimming isn't hard?
Did you know that Kangaroos are excellent swimmers? Kangaroos don't usually swim, but when they do, they're actually quite good at it! They use their powerful back legs in a doggy paddle style. It's an interesting quirk of a kangaroo's biology that they can only use their legs independently for motion when swimming.