Before we can go to the pool and learn how do you float in the water, we should understand how it works. Floating makes your body stay on the surface, thanks to the buoyancy force.
Since we are still on Earth, gravity pushes any object inside the water. Suppose the object is your body. Your body submerged inside of the water displays the water with its mass. The water you displace with your mass will push your body toward the surface. ⛴
The more water you displace with your mass, the easier it is for you to stay close to the surface. You can displace the most water by fully submerging the body inside.
Since we are trying to be buoyant while floating on the back, we should let our entire body stay inside the water, excluding our face. We need to keep our faces dry for a couple of reasons. One, so we can breathe. Two, to keep the water out of our noses.