Do the oceans have real boundaries?

While there’s only one world ocean, maps recognize five different oceans and several more seas worldwide. How can we distinguish them? Are there “real” borders or boundaries between them? Do the waters of different oceans really get mixed?

Imagine for a moment that you spend your holidays in the sunny coast of Croatia. You are swimming in the crystal waters of the Adriatic sea when you start thinking about if you are having a bath also in the waters of the Mediterranean sea.
Are they the same? If so, why do they have different names? And, if they are not the same, why? Is there any physical border or boundary between two distinguishable bodies of water?

Let’s start from the scratch. The same way all continents of the Earth were once a single huge continent named Pangaea, all the oceans belong to a single world ocean (Panthalassa), as the waters of all of them —and the rest of the smaller seas— are connected through straits or just open currents all over the globe. There is only one case where the continental drifting left one sea enclosed and so its waters disconnected to the world ocean, that is the Caspian sea.

The Caspian sea (in the middle) is not a lake, but a portion of the world ocean that, due to tectonics, became enclosed inside the asian continent (image by Globe Master 3D)

Basically, the easily distinguishable areas where the world ocean seems divided by the continents, we name them as oceans, in a similar way we name continents as well. So today we tag up to five different oceans: Artic, Atlantic, Pacific, Indian and South. They are connected, they meet at some point. Atlantic and Pacific ocean meet at the cape of Horn through the strait of Magellan (only there?) and the Indian and Atlantic ocean meet at the cape of Agulhas (and not in cape of Good Hope).

The Cape of Good Hope (near Cape Town, South Africa) is not the frontier between the Atlantic and Indian oceans, but the Cape Agulhas (center bottom of this map) that is eastern, southern from it. (imagen by Johantheghost)

Also, as a separated entities, but still connected to the world ocean, we have the smaller seas like the Mediterranean, the Caribbean or the Red sea (but not the Dead sea, that is a salt lake) that can contain even smaller seas at the same time. So let’s go back to the beginning of this article: are we swimming in the Adriatic sea or the…