The Aqua planet was a planet home to the aquamonsters. The planet was partly dry, but there were oceanpheric currents of invisible liquid which moved around the surface, usually carrying aquamonsters within them.
When the First Doctor, Ian Chesterton and Barbara Wright landed on the aqua planet, Ian noted fish flying. It turned out that they were actually swimming in invisible liquid. The oceanpheric current caught up to them but quickly moved away. (COMIC: Dr. Who on the Aqua Planet)