Beginner 8 minutes
Oceans, seas, gulfs, and straits: the geography that organizes the blue planet
How water gets named, what shape the seafloor takes, and why that decides where you dive
All the water in the sea is connected, so dividing it into oceans and seas is a human convention. But it's not an arbitrary one: the names follow the shape of the seafloor fairly closely, and the shape of the seafloor decides where there's current, where there's life, and where it's worth diving.
In this article you'll see what sets an ocean apart from a sea, a gulf, a bay, or a strait, what the seafloor profile looks like from the shore to the trenches, why straits concentrate current, and what each of those stretches means for a diver.
Geography tells you in advance what to expect from a site: whether there'll be current, whether the bottom will drop off vertically, whether the water will be cold or warm, and whether life will concentrate at one particular spot.
How water gets named
The terms have criteria behind them, even if historical usage blurs them.
An ocean is one of the great bodies of water that separate continents: the Pacific, Atlantic, Indian, Arctic, and the one that surrounds Antarctica, whose recognition as an ocean in its own right became widespread relatively recently. Between the five, they cover most of the planet's surface.
A sea is a smaller division, usually partly enclosed by land or by arcs of islands and connected to an ocean. Some, like the Mediterranean, are almost landlocked and communicate only through a narrow passage; others, like the Coral Sea, are open sectors with their own name.
A gulf is a wide entry of the sea into the land, and a bay is a smaller, usually more sheltered entry. The distinction between the two is one of scale and tradition, not strict definition: there are small gulfs and huge bays.
A strait is a narrow passage connecting two larger bodies of water, and a channel is an elongated passage between two landmasses. These are the most interesting structures from a diving standpoint, and the article dedicated to them owes more to physics than to geography.
There are also terms for particular situations: an inland sea with no outlet to the ocean, a fjord carved by a glacier, an atoll enclosing a lagoon. Each implies specific water conditions.
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