Just a quick look at who they are and what they're made of. The fact that I do polar work leads to me taking a somewhat different accounting of the 'who' side than low and mid-latitude oceanographers tend to. So, for me, the oceans are: Pacific, Atlantic, Indian, Southern, Arctic. The Southern and Arctic oceans are not infrequently neglected. The Arctic can reasonably be called a sea rather than an ocean; that is, it is a body of water that has limited inflow and outflow (the Bering Strait on the Pacific side and Fram Strait on the Atlantic side).
The Southern ocean truly is oceanic rather than sea-like. We polar types consider it separate from the other oceans because of the Antarctic Circumpolar Current. South of this, the Southern Ocean waters are mixed pretty well with each other and are not much like the waters to the north. North of there, the other oceans have greatly limited transport with the Southern Ocean. Currents tend to flow from one of the other three to another of the others, rather than in to the Southern Ocean. Since the Antarctic Circumpolar Current, like all currents, is a dynamic feature, it moves. This means that the boundary location and size of the Southern Ocean (and, conversely, the Pacific, Atlantic, and Indian) fluctuates somewhat in time.
The elements in the ocean, as for most geophysical systems, are mostly a fairly select subset of the periodic table. Although there are over 110 elements known now, only 8 account for most of the ocean. If we take a kilogram of sea water (1000 grams, remember), then there are about:
858 grams Oxygen (O)
107 grams Hydrogen (H)
10.3 g Sodium (Na)
19.5 g Chlorine (Cl)
1.3 g Magnesium (Mg)
0.9 g Sulphur (S)
0.4 g Potassium (K)
0.4 g Calcium (Ca)
Aside: Sodium and Potassium's chemical symbol comes from their Latin names 'Natrium', and 'Kalium', respectively. Their symbols do make sense, at least if you know the right second language.
So, 8 elements, 8 symbols to remember out of the 110+, and you've got well over 99% of the ocean's content. Even better is that the composition of the ocean varies by very little throughout, at least for the major elements. Very scarce elements, like Iron (Fe -- Ferrum) do vary substantially, but they're so rare this doesn't affect the ocean's dynamics.
If we think instead of molecules, the main components are water (H2O), table salt (NaCl), additional salts (KCl, CaCl), and Magnesium Sulphate (MgSO4, epsom salts). Of these, water is the overwhelming majority. Table salt is most of the rest. Viewed in terms of its bulk composition, the ocean is fairly simple. But if we think about living things in the sea, it gets complicated very quickly. Living things depend on elements and chemicals which are very scarce.
The oceans, all together (and however we name them), occupy about 70% of the surface of the earth. This is one reason oceanographers take an interest -- it is most of the surface of our planet. (Calling it 'Earth' is poor naming, we feel. It should be 'Water', or 'Oceania' or something like that.) The average depth of the ocean is about 3730 meters (about 12,200 feet), farther below sea level than almost all land areas are above sea level (land averages about 800 meters, 2600 feet, above sea level; by the time you get to 12,000 feet above sea level, you're on the side of a good mountain).
Later, we'll get more detailed about the ocean. But here's a start on names.