Sea vs Ocean: The Key Differences

Written by Krishna Maxwell
Updated: January 24, 2023
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Key Points

  • Seas are small portions of an ocean. They are located in areas where ocean and land meet.
  • Seas areas can be further broken up into multiple smaller seas that attach to separate shores
  • In general, seas are partially enclosed by land as opposed to larger oceans which surround large land masses.
  • An ocean can have separate seas.

In science, there is usually a technical term for everything. The somewhat nebulous definition of a sea is one of the things that makes the difference between a sea and an ocean so interesting. The terms are somewhat interchangeable in many ways. This may be because the original definition of seas and oceans predated the rediscovery of science in the Middle Ages.

When sailing ships were just beginning to cross oceans, there was no real distinction between an ocean and the sea. These antiquated nautical traditions have survived to this day. As a result, the terms sea vs. ocean are more generalized rather than highly specific. The original hazy definitions have therefore been allowed to continue into modern times and remain somewhat nebulous in their differentiation. Custom, in this case, has proven stronger than the determined scientific definition. Let us have a look at this comparison of “sea” vs “ocean.”

Comparing Sea vs Ocean

As generally understood today, all oceans are larger than seas. It is also often the case that seas are considered to be components of a larger body of water. This is frequently, but not always, an ocean to which the smaller seas belong. In the most obvious case, the Mediterranean Sea is not an ocean, yet various parts of it are also considered to be seas in their own right.

The Tyrrhenian Sea is the portion of the Mediterranean Sea that washes up against the western and northwestern coasts of Italy. Over on the other side of the Italian boot, the body of water between Italy and the Balkan peninsula is generally known as the Adriatic Sea. Further still, the part of the Mediterranean on the other side of the Balkan peninsula is known as the Aegean Sea.

Out in the wider oceans themselves, it is possible to find many seas listed as being included as a similar type of subunit in their overall oceanic domain. The South China Sea is much in the news these days. Along with the Yellow Sea and the Sea of Japan, these are all part of the great Pacific Ocean yet retain their own distinct local identity as places along the Chinese coast.

The Key Differences between Sea vs Ocean

As mentioned above, size matters in terms of the definition of seas and oceans. Yet other differences also differentiate the two. Oceans are considered to be saltwater while seas are often considered to be saline water or less salty water. This is due to the proximity of seas to the landmasses of the earth. Freshwater rivers dump their output into the seas off the coast. This dilutes the oceanic salt level to some degree.

Access also matters. The wider waters of the world are considered to be five interconnected oceans, although this traditional definition is becoming modified to some degree. Some seas, such as the Aral and Caspian Seas, are completely divorced from any connection to the oceans and are thus also able to be defined as saltwater lakes such as the Great Salt Lake in Utah.

Other seas are notable for their limited number of connections to the wider oceanic mass. The Baltic Sea, for example, only has one outlet to the wider Atlantic Ocean. The Black Sea only has one narrow outlet to the Mediterranean Sea, which in turn only has one single natural outlet to the Atlantic as well. On the other end of it, the Suez Canal offers a manmade egress to the Red Sea, which in turn only has one opening out into the Indian Ocean. In all cases, seas are noted for their proximity to land.

Bays & Gulfs vs Seas & Oceans

The different bodies of water can get quite confusing. Bays and gulfs are part of a sea or an ocean but differ in their geographic anatomy. For instance, bays formed where the land curves inward around a broad, recessed coastal inlet. Bays are typically areas where ships can find sheltered areas that break to full power of large waves.

Gulfs have a more enclosed entrance and their inlets are deeper. So, all bodies of water with different characteristics of land formations and water constitution.

All of the terminology associated with the naming conventions of large bodies of water arose from the needs of shipping. We still rely on a variety of shipping vessels to move food and goods across the world. While our ships have gotten larger and no longer rely on wind power, the nautical terms used 500 years ago are still applicable today.

Up Next…

The oceans and seas of the world are vast. Mysteries abound are out there, so find out more about what we do know about oceans:

The photo featured at the top of this post is ©

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About the Author

Krishna is a lifelong animal owner and advocate. She owns and operates a small farm in upstate New York which she shares with three dogs, four donkeys, one mule, and a cat. She holds a Bachelors in Agricultural Technology and has extensive experience in animal health and welfare. When not working with her own animals and tending her farm, Krishna is helping other animal owners with behavior or management issues and teaching neighboring farmers about Regenerative Agriculture practices.

FAQs (Frequently Asked Questions) 

What are the 7 Seas and the 5 Oceans?

This is not an easy question to answer. As a rule, the 5 oceans are considered to be the Antarctic, Atlantic, Indian, Pacific, and Arctic Oceans.

As for the 7 seas, this is a term that was never very exact and has different definitions during different historical eras. At the dawn of Western Civilization when humanity lived in a Mediterranean-centric world, the 7 seas were traditionally considered to be the Mediterranean, the Adriatic, the Aegean, the Black, the Red, the Caspian, and the Arabian Seas. In some slightly different lists, the Aegean was dropped and the Persian Gulf was added in its stead.

As Western man moved out from the original homeland, the definition of the 7 seas changed as new bodies of water were discovered.

Once seafaring became possible over the whole expanse of the globe, the term “7 seas” took on a more expansive meaning to suggest the planet’s body of saltwater in its entirety. It became a term of art rather than a scientific definition. In modern times, the 7 seas are generally defined as the Arctic, North Atlantic, South Atlantic, Indian, North Pacific, South Pacific, and Antarctic Oceans.

One change in terminology is gradually taking hold wherein the Antarctic Ocean is now frequently being called the Southern Ocean.

What are the 5 main seas?

This term can refer to the 5 modern oceans listed above. It can also refer to the 5 large bodies of water that share only limited connectivity (or none) with the oceanic mass. These would be the Baltic, Mediterranean, Black, Caspian, and Red Seas. The White Sea in far northern Russia would also seem to qualify as a similar entrant under this standard but is generally not included.

Which is deeper: the ocean or the sea?

The oceans are far deeper than any of the seas. Ocean depths can go as low as 36,000 feet below sea level in the Marianas Trench in the central Pacific. The Mediterranean’s deepest point is about 15,000 feet below sea level. Seas however tend to be much shallower on average than the oceanic medium depth.

Where are the 7 seas and 5 oceans?

They are everywhere. In fact, you are more than twice as likely to find a spot on the water as you are to fix a location on land. Overall, the world is more than 70% covered in water. Geographically speaking, it is thus more correct to talk about the limited portions of land wedged in between the vast masses of water that comprise the oceans of the world.

With that said, the mighty Pacific Ocean, both North and South, is west of the Americas and east of Asia. The Atlantic Ocean, both North, and South, is west of Africa and Europe and east of the Americas. The Indian Ocean is west of the westernmost boundary island chains of the Pacific and east of the continent of Africa. Down below these central oceans, the Antarctic, or Southern, Ocean surrounds the continent of Antarctica and washes up against the southern fringes of the Atlantic, Pacific, and Indian Oceans. Up at the top of the map, the Arctic Ocean washes the roof of the world and borders Russia, Norway, Greenland, Canada, and Alaska.

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