Simply floating in water might look easy, but it's actually pretty difficult if you don't have a flotation device, and treading water is an essential safety skill for anyone who plans on spending time on or near the water, well, ever. Here's the best way to do it for as long as possible in an emergency situation.
Who knows more about staying afloat in water longer than the US Navy (or any Navy, really)? Once you've plunged into the pool, lake, pond, or ocean, PR1 Michael Pasquale at the Naval Survival Training Institute suggests you start by trying to keep the water at your chin level, with your head always above it.
Your body should be positioned as if you were sitting on a stool, with your head slightly tilted back.
Now, scull your hands back and forth, or move your hands, palms down, in a circular motion under the surface of the water. While you do that, use a modified frog kick -- not a scissor kick -- to help keep your head above water as well. A modified frog kick involves bringing your knees up towards your chest with your legs out, then kicking your feet down before repeating the process.
Continually repeat all of these movements until you can be rescued or find a flotation device.