The author writes that Earth's oceans are similar to

Lakes, pond, rivers, beaches are what the ocean of the Earth have been compared to.

Explanation:

Oceans are the bodies of water that compose mostly the hydrosphere of the planet Earth. They are huge bodies of water. There are five oceans in the whole world.

The name of those oceans are the Pacific ocean, the Arctic ocean, the Atlantic ocean, the Indian ocean and the Southern ocean. Out of these oceans the Pacific ocean is the largest and the deepest ocean and the smallest ocean is the Arctic ocean out of all the oceans found on the surface of Earth.

They are the largest bodies of water where the rivers coming from different part of the world come and meet in the various oceans of the world.

But the author here compares the oceans of the planet Earth with the Beaches, rivers, ponds and lakes. They are comparatively much smaller than the body of an ocean but still the author makes a comparison of these with the oceans.