Yeah, I'm pretty sure that's the only way to swim. If you consider yourself a decent swimmer, I wouldn't sweat the test. That being said, feel free to add swimming to your workouts. It's a great workout, and who knows, you might end up going to scuba school down the road, or accidently wind up in water during training or combat.
Also, I think you are a little confused about Ranger School - are you already in the Army? If so, nevermind.