You can stand up straight in water that is over your head, and from the middle of your breast upward you will not be wet.

But you can not remain so.

The water will soon float your feet to the surface.

You can not swim on your back and make any progress of any consequence, because your feet stick away above the surface, and there is nothing to propel yourself with but your heels.

If you swim on your face, you kick up the water like a stern-wheel boat.