Water masses cover about 71% of the surface of the world and can be divided into four major categories (excluding lakes):
- Oceans. Very large continuous bodies of salt water that are bounded by continents. They can be latitudinally divided for convenience, such as for the Atlantic and Pacific Oceans (North and South). They are longitudinally divided by the narrowest segment from a southern continent and Antarctica.
- Seas. Bodies of salt water that are partially or fully enclosed by land. The Mediterranean is considered a sea because of its full enclosure.
- Gulf and bays. Oceanic or sea inlets with gulfs considered larger and deeper than bays, but gulfs have narrower entrances.
- Channels, straits and passages. Narrow bodies of salt water connecting oceans. Straits are comparatively short, channels of medium length, and passages are long.