Question

why is the earth called the watery planet

Answer 1

because 2/3 of the planet is made of water and only 1/3 is land

Answer 2

If you look down at our planet from outer space, most of what you see is water; 71% of the planet's surface is covered by ocean and it is because of this that the Earth is sometimes called "the water planet". Only about three-tenths of our globe is covered with land.

The ocean wraps the globe and is divided into four major regions: the Atlantic Ocean, the Pacific Ocean, the Indian Ocean and the Arctic Ocean. Some scientists consider the waters around Antarctica to be a separate, fifth ocean as well. These oceans, although distinct in some ways, are all interconnected; the same water is circulated throughout them all. 
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​Seventy-one percent of the Earth's surface is ocean.

If all the continents were crammed into one corner of the Earth, the vast extent of the world ocean could easily be seen. In reality, of course, the continents are not bunched together as shown in the figure above, but instead are spread out over the entire Earth's surface. Most oceanographers, however, believe that a long time ago in the Earth's geologic history all of the continents were once grouped closely together in much the same manner.