Sea vs. ocean. Is there any difference, or are they simply two words we can use to describe the saltwater that covers 70.8% of the surface of the Earth?
People certainly use the two words interchangeably in everyday use, and your preference may depend on your country. However, the main difference between the ocean and the sea is their proximity to the land.
Oceans are enormous and span the vast distances between our planet’s continents.
In contrast, the seas are divisions of the ocean next to the land and are often partially enclosed by it.
We will also discover that oceans are much larger in area than seas and have a deeper average depth. There can also be differences in temperature and salinity between them, but that tends to have more to do with the specific location.
What Is an Ocean and What Is a Sea?
Oceans
The ocean is the vast open aquatic system that covers 70.8% of our planet and contains about 97% of the total water supply.
The worldwide ocean is divided into five ocean basins – with, in order of size, the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Southern Ocean, and the Arctic Ocean.
These five named ocean regions are all connected together and are further divided into smaller designated bodies of water, most notably the seas.
The Russian oceanographer and map maker Yuly Shokalsky came up with the phrase “worldwide ocean” to emphasize that all saltwater on Earth is one continuous body, no matter what it’s called.
Seas
Seas are the most significant bodies of water where the oceans meet the land and are often enclosed, at least partially, by the coast.
Around the world, many tens or even hundreds of ocean areas are called seas. Smaller regions of a sea are called gulfs, bays, straits, and other terms depending on location.
The seas themselves are divided into several sub-categories:
Marginal Seas – Divisions of the ocean partially enclosed by land but mostly connected to the open ocean. For example, the North Sea between Great Britain, Norway, Denmark, Germany, the Netherlands, and Belgium, is a marginal sea of the Atlantic Ocean. Other marginal seas include the English Channel, the Caribbean Sea, and the Coral Sea.
Mediterranean (or Enclosed) Seas – Marginal seas heavily enclosed by land with a limited exchange of water with the outer oceans. Examples include the namesake Mediterranean Sea and the Red Sea.
Inland Seas – Large salty water bodies “nearly or completely surrounded by land.” with just a narrow connection to the ocean. For example, the Baltic Sea or the Marmara Sea.
The exception is the Sargasso Sea, a large area of the North Atlantic Ocean far away from the east coast of the United States. This “sea” is unique and is defined by the ocean currents surrounding it rather than any land.
It’s worth noting that there is no firmly accepted concrete definition of sea among oceanographers. In many ways, seas or oceans are just names that have been given and have remained in use throughout history.
Indeed, the Glossary of the Mapping Sciences notes that “the boundaries between the oceans and seas are often indistinct and more a matter of arbitrary definition and convention than a physical difference.”
The Key Differences Between an Ocean and the Sea
Quantity – How Many Oceans and Seas are There?
Counting oceans is pretty straightforward. As we have mentioned, many oceanographers like to consider the worldwide ocean as a whole entity. However, it’s accepted that there are five defined ocean basins: the Pacific, Atlantic, Indian, Southern (Antarctic), and Arctic Oceans.
When it comes to counting seas, it gets a lot trickier.
Many seas exist as traditional localized names only and are called something else internationally. A sea is a subdivision of an ocean, but then you can discuss whether it should be called a sea, a gulf, a strait, or a bay.
The International Hydrographic Organization names 79 bodies of water as seas in their publication ”Names and Limits of Oceans and Seas.” Other lists show more than one hundred areas called seas.
Local names change with perspective and history. After all, it was once common to use the “Seven Seas” to describe all the seas of the known world. But this term has changed significantly in scope through the ages.
Size
Oceans are the largest bodies of water on Earth, while seas are much smaller divisions called second-order sections.
The largest of the oceans, the Pacific Ocean, covers 168,723,000 km2 (65,144,315 mile2) which is 46.6% of the total of all the planet’s oceans.
In comparison, the largest named sea, the Philippine Sea, a marginal sea of the Western Pacific Ocean, covers a relatively small 5,695,000 million km2 (2,198,852 mile2).
The smallest body, specifically called a sea, is frequently named as the Adriatic Sea at 138,000 km2 (53,282 mile2). However, the Sea of Marmara in Turkey can claim the crown at just 11,350 km2 (4,380 mile2). Although this inland sea is often regarded as being part of the much larger Mediterranean sea, meaning the choice isn’t straightforward.
Depth
Are seas deeper than oceans? No, the deepest parts of the oceans are deeper than the seas.
At its deepest point, the Mariana Trench in the western Pacific, the ocean reaches an incredible 10,984 meters (36,037 feet) below the surface.
On the other hand, the deepest area named a sea, the Coral Sea, off the northeast coast of Australia, has a maximum depth of 9,140 meters (29,990 feet).
To better compare, we should consider average depths; the entire Pacific Ocean comes out at 3,970 meters (13,024 feet). The Coral Sea measures 2,394 m (7,854 ft) on average.
These differences might not seem huge, but the Coral Sea is exceptional. For example, the Mediterranean Sea has an average depth of 1,500 meters (4,900 feet), while the average depth of the North Sea is only 95 meters (312 feet).
The typical sea is still shallow due to the geology beneath them. Most seas are above the submerged continental shelf, stretching out between only a few kilometers or miles to as many as hundreds offshore.
The continental shelf makes up about eight percent of the total seafloor area, and most of it, and accordingly, many seas, have a maximum depth of 100–200 meters (328-656 feet).
The average depth of all the oceans is far greater at 3,688 meters (12,100 feet.)
How Oceans and Seas Were Formed
The Earth’s first oceans were formed many billions of years ago when the planet cooled and water vapor in the atmosphere condensed in vast amounts to fill the low basins on the surface.
Originally the ocean existed as one “superocean” that surrounded the supercontinent Pangaea.
As Pangea’s tectonic plates separated and the landmasses broke apart, the modern oceans we know today eventually came into being.
The seas were also formed during this process. However, the coastal shapes we know today have been created by multiple influences, including weather and erosion.
The enclosed seas were typically formed when a large landmass, such as a continent, blocked the water flow from one of the oceans or when a plate shifted and rose upwards. Others have been created as plates moved apart and opened wide gaps that the sea filled.
Aquatic Life
The seas are generally more sheltered and have a greater variety of plant and animal life in higher concentrations than the ecosystems of the open oceans.
For example, seas are home to spectacular coral reefs, which are said to be home to 25 percent of all marine life, including over 4,000 species of fish.
On the other hand, Oceans are more exposed to the elements, and their ecosystems are often characterized by open water or extreme depth.
However, the comparison isn’t completely one-sided, as there is life in the deep sea, even way beyond the depths that sunlight can penetrate.
The ocean is a complex environment that is still not fully understood. Despite centuries of research, many areas are left to be fully explored.
With so much still to learn about this vast and fascinating part of the planet, it’s entirely possible that new species still await discovery.
However, we can say that the most concentrated and widest variety of marine life lives reasonably close to land, and that means in areas called seas.
Temperature
The temperature in the seas varies widely depending on how close they are to the equator. But deeper ocean waters will always be cooler than shallower seas in like-for-like areas.
The deeper the water, the more stable the temperature, and while shallow tropical waters may vary by 10°C (50°F) through the year, the deep ocean will keep a constant value.
Salinity (Salt Content)
Both seas and oceans are saltwater bodies, but the salinity levels can vary quite significantly.
Seas next to the coast may have lower salinities than the surrounding ocean due to the addition of freshwater from rivers and other land runoff.
However, in the Red Sea, the salinity is some of the highest found anywhere due to its confined geography and hot arid climate.
Similarly, the Mediterranean Sea has a higher-than-average salinity as it doesn’t receive that much rain to compensate for evaporation. It is reasonably enclosed from the Atlantic Ocean, which would otherwise dilute it.
Ocean salinity is generally pretty constant across similar regions. Tropical regions tend to be saltier than temperate areas as there is more evaporation and less rain.
Economic and Cultural Significance
Humans have been traveling on water from their lands for many thousands of years, and it’s fair to say that, up until reasonably recently, they did this on the closer-to-shore seas.
In modern times, many human activities, including fishing, recreation, travel, extractive industries (drilling and mining), and power generation, still occur in what we tend to call seas.
The open oceans are mostly left for transporting goods between the world’s seaports. Mainly across the Atlantic or the Pacific.
Environmental Threats and Conservation Efforts
Seas close to the land are usually the first to be affected by pollution or activities like overfishing.
However, the open ocean isn’t immune to the damage caused by humans.
For example, circular ocean currents have caused human debris to collect far offshore as the Great Pacific Garbage Patch.
In addition, as humans have developed various technologies, parts of the open ocean have been used as an “out of sight, out of mind” dumping ground for chemical and even nuclear waste.
In this context, while the seas are close to home and incredibly important, they are parts of the whole worldwide ocean, and we must protect all of them.
Legal Differences Between the Ocean and the Sea
The oceans and the seas are treated the same in maritime law, where one term is used universally.
The United Nations Convention on the Law of the Sea (UNCLOS) provides a legal framework for all maritime and marine activities.
As it describes “all issues relating to the law of the sea,” the UNCLOS clarifies that all of the ocean is “sea.” It even explains the open water beyond a country’s coastal waters as “the high seas.”
So, if you’re discussing maritime law, everything is the sea. Even the ocean!
Seas and Oceans in Everyday Use
How you describe the water in front of you as you look out from the beach could depend on where you live.
For example, in the United States, it’s normal for people to call all saltwater “the ocean,” but in the United Kingdom, British English speakers will call it all “the sea.”
Put another way, unlike in the US, in England, you go swimming in “the sea” and not “the ocean.”
It may well just be that Europeans are used to the smaller local waters that have been named seas historically, while in the United States, where everything tends to be on a larger scale, the “bigger” word has become the norm.
Conclusion
When discussing sea vs. ocean, it’s essential to understand that we’re talking about large bodies of water that are all joined together as one continuous worldwide ocean.
The difference between the sea and an ocean can just be how they’re known in different parts of the world. In the UK, almost everyone calls all saltwater the sea, while in the US, it’s far more usual to call everything the ocean.
However, outside of common everyday use, while seas and oceans are both vast bodies of saltwater, they have several key differences that set them apart.
Oceans are the complete and giant areas of water stretching across the planet between the continents.
Seas are subdivisions of an ocean in areas bordered by the land. They can even be partially or almost entirely enclosed.
Seas are also smaller and almost always shallower. However, they can contain much more marine life than their deeper parents.
Whether we call the salt water that makes up the blue part of our planet the sea or the ocean, it is essentially just a name. What is far more critical is that our life-giving worldwide ocean and its inhabitants are given the protection they need from human activities before it is too late.
British-born Dan has been a scuba instructor and guide in Egypt's Red Sea since 2010.
Dan loves inspiring safe, fun, and environmentally responsible diving and particularly enjoys the opportunity to dive with sharks or investigate local shipwrecks.
When not spending time underwater, Dan can usually be found biking and hiking in Sharm's desert surroundings.