What is the difference between the sea and the ocean?

Size, physical boundaries, depth... Several criteria distinguish seas from oceans. A distinction that is based more on conventions than on objective characteristics! When Vasco Nuñez de Balboa crossed the Isthmus of Panama in 1513 and entered unknown waters, he called it the South Sea. Five years later Magellan, having sailed there in calm weather, called it the Pacific Ocean... A name that did not become official until the late 19th century. That's one way of putting it: seas and oceans are first and foremost conventions. They are human inventions. Hence the difficulty in distinguishing them clearly, or even justifying them. At present, there are several criteria. The first is the most intuitive: what distinguishes the sea from the ocean is, first of all, the relation of scale. By definition, the ocean is larger. The smallest, the Arctic Ocean, is 14.75 million square kilometers, while the largest sea, the Arabian Sea, covers 3.8 million square kilometers. The oceans border the continents The seas have a smaller surface area, but what else? Oceanographers also consider the nature of the coast, the depth of the bottom, and the salinity of the water. Conventionally, the term "ocean" refers to the largest expanses of salt water bordering the continents. There are five: the Pacific, Atlantic, Indian, Arctic, and Southern.
The seas fall into two main categories. On the one hand, there are coastal seas that run along the contours of the oceans and are bounded by peninsulas, islands, or shoals. The Caribbean Sea, for example, borders the Atlantic Ocean and is surrounded by a number of islands. Keep in mind, however, that sea does not necessarily mean "shallow water"! The coastal seas can be very deep, over 2,000 meters, as in the Norwegian Sea (the deepest is 3,970 meters) or the Coral Sea (maximum 9,174 meters, located between Australia and New Caledonia). But these seas can also be located on the continental shelves, which are shallower than the oceanic plates and therefore do not exceed 200 meters in depth: this is the case of the English Channel or the North Sea. The second main category, the enclosed seas, communicates with the ocean only through a strait and is almost completely surrounded by land. Like the Mediterranean Sea, which is connected to the Atlantic Ocean by the Strait of Gibraltar. As a result, in contrast to coastal seas, their hydrological characteristics are very different from those of the ocean. Lacking access to the ocean, they are subject to much evaporation and are often more salty than the world's oceans (five oceans communicating with each other). While the latter has a salinity of 35 g/l, in the Mediterranean it ranges from 38 to 39.5 g/l, rising to 41 g/l in the Red Sea and peaking at 275 g/l in the Dead Sea! However, in the Baltic Sea, where there are many rivers, it drops to 5-10 g/l. The final difference is that the seas are more biodiverse than the oceans. The Mediterranean Sea, for example, is home to 4 to 18% of the world's marine species, while this region accounts for less than 1% of the world's ocean surface!