der Ozean
ocean
One of the large world oceans that surround the continents.
Die Titanic sank auf ihrer Fahrt über den Atlantischen Ozean.
The Titanic sank during its voyage across the Atlantic Ocean.

ocean
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ocean
One of the large world oceans that surround the continents.
Die Titanic sank auf ihrer Fahrt über den Atlantischen Ozean.
The Titanic sank during its voyage across the Atlantic Ocean.

The preposition changes depending on the context: you swim 'im Ozean' (in the ocean; dative), you travel 'über den Ozean' (across the ocean; accusative), and you travel 'an den Ozean' (to the ocean; accusative).
Viele Schiffe überqueren den Atlantischen Ozean.
Many ships cross the Atlantic Ocean.
'Ozean' (ocean) is often used with the official names of the world's oceans: 'der Atlantische Ozean' (the Atlantic Ocean), 'der Pazifische Ozean' (the Pacific Ocean) or 'Pazifik' (Pacific), and 'der Indische Ozean' (the Indian Ocean).
Der Pazifische Ozean ist der größte Ozean der Welt.
The Pacific Ocean is the largest ocean in the world.
You can recognize the figurative meaning by the fact that 'Ozean' (ocean) is combined with a noun that has nothing to do with water. It often appears in the form 'ein Ozean aus/von ...' (an ocean of ...).
Die Rede löste einen Ozean von Beifall aus.
The speech triggered an ocean of applause.
In the figurative meaning, you often find poetic expressions such as 'ein Ozean der Gefühle' (an ocean of emotions), 'ein Ozean der Stille' (an ocean of silence), or 'ein Ozean an Möglichkeiten' (an ocean of possibilities).
Nach der Trennung versank er in einem Ozean der Traurigkeit.
After the breakup, he sank into an ocean of sadness.
lake
'der See' and 'der Ozean' both refer to large bodies of water, but they are not the same size and are located differently. A lake lies inland and is surrounded by land. An ocean is a huge world ocean between continents. Learners may confuse the words because both have to do with water and nature. When talking about swimming, hiking, or a body of water in a landscape, 'der See' is usually the right choice. When it is about world oceans, ships, or continents, 'der Ozean' is correct.

Das Schiff überquert den Ozean auf dem Weg nach Amerika.
The ship is crossing the ocean on its way to America.
Am Wochenende gehen wir an den See und schwimmen dort.
At the weekend, we’re going to the lake and swimming there.