Key difference: Oceans are a vast body of salt water that cover almost three fourths of the earth's surface. A bay is a body of water covered by land on three sides.

Oceans and bays are both bodies of water. Water bodies make up the hydrosphere that covers 71% of the Earth. However, there are some differences in what is called an ocean and what is termed as a bay.

The Earth’s hydrosphere, i.e. all the water bodies on earth, makes up the World Ocean. The World Ocean is then further distinguished into 5 oceans, the Pacific, Atlantic, Indian, Southern (Antarctic), and Arctic. These oceans are in decreasing order of size, with the Pacific Ocean being the biggest, and the Artic Ocean being the smallest.

There is no official description for an ocean, except for a vague definition stating a vast body of saline water. Oceans cover almost three fourths of the earth's surface. They are the largest body of water on the planet and contain approximately 230,000 types of marine life. The depth of an ocean ranges between anywhere from 13,000 feet to 35,000 feet.

A bay, on the other hand, is a body of water surrounded by land on three sides. A bay may also be called a gulf, a sea, a sound, a bight, a cove, a fjord or a ria. The largest bay in the world is the Bay of Bengal.

As compared to other bodies of water, bays have less wave and wind activity than the water outside the bay. They also tend to have sandy beaches and are shallower than the ocean. The mouth of a bay can be narrower than the bay or can be much wider, opening out into the ocean. Bays can also be adjacent to other bays.

There are various ways in which bays are created, including as a result of continental drift, via glacial and river erosion, due to folding of the earth's crust, coastal erosion by waves and currents, and can even be man-made.

Bays usually tend to contain fish and other sea creatures, and due to their lower wave and wind activity, they provide a place for fishing. Since they are also shallower than the ocean, they are a great place for safe anchorage and are usually used as ports.

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