Natant is a formal way to describe something or someone that's floating. When you finally learn how to float on your back during a swimming lesson, you can say, "Hey, I'm natant!"
You'll nearly always find natant and other words that share the Latin root natare, "to swim," used in a scientific context. A natant decapod, for instance, is a shrimp. You can substitute natant for "swimming" or "floating" in everyday speech, but not many people will understand what you're talking about. But give it a try if you want to sound fancy: "I wish I had a pool, so I could be natant all day long."