An ocean is a large body of water on a planet, larger than a sea. Many oceans, such as those on Earth, contain salt water. Often where the water meets the shore, beaches can be found.
The seas of Earth are blue in color, whereas the seas of Bajor are green and the seas of Trill are purple. (DS9: "Past Tense, Part I")
The planet Argo is known as an "ocean-planet", as its surface was almost entirely covered by water. (TAS: "The Ambergris Element")
Around 100,000 years ago, an unknown species in the Delta Quadrant used a kinetic transfer system to move the oceans of a planet into space, creating what the Moneans call the Waters. (VOY: "Thirty Days")
In 2024, preparations were being made for seafloor mining operation in the territory of the Pan-Caribbean government. (DS9: "Past Tense, Part II")
In the Julian Bashir, Secret Agent holoprogram, Hippocrates Noah's plan for world domination involves releasing "millions of tons" of lava, that would cause Earth's tectonic plates to subside. This would result in the planet's continents to be flooded by the oceans, save for his own personal refuge atop Mount Everest. (DS9: "Our Man Bashir")
In 2374, a Jem'Hadar fighter commanded by Benjamin Sisko crashed into the sea of a planet in Cardassian space. (DS9: "Rocks and Shoals")