See vs. Meer
While both German words Meer and See can be translated as sea, they are not interchangeable. The difference depends on the type of water body and, crucially, on the grammatical gender of the word See.
See
A1The word See has two different meanings depending on its grammatical gender. Der See (masculine) means the lake. Die See (feminine) means the sea, often in a more poetic or nautical context, referring to the high seas or a vast, open expanse of water.
Wir schwimmen im Bodensee.
(We swim in Lake Constance.)
Der See ist heute sehr ruhig.
(The lake is very calm today.)
Ein alter Seemann verbringt sein Leben auf See.
(An old sailor spends his life at sea.)
Die stürmische See machte die Überfahrt gefährlich.
(The stormy sea made the crossing dangerous.)
Lass uns ein Boot mieten und auf den See hinausfahren.
(Let's rent a boat and go out on the lake.)
Viele Schiffe sind auf hoher See verschollen.
(Many ships have been lost on the high seas.)
Meer
A1The word Meer (neuter, das Meer) is the most common and general term for sea or ocean. It refers to the large saltwater bodies that cover most of the Earth. This is the word you would typically use when talking about a beach vacation or a specific sea like the Mediterranean.
Wir fahren im Sommer ans Meer.
(We are going to the sea in the summer.)
Das Mittelmeer ist für sein warmes Wasser bekannt.
(The Mediterranean Sea is known for its warm water.)
Ich liebe es, das Rauschen des Meeres zu hören.
(I love to hear the sound of the sea.)
Delfine und Wale leben im Meer.
(Dolphins and whales live in the sea.)
Die Kinder bauen eine Sandburg am Meer.
(The children are building a sandcastle by the sea.)
Die Küste fällt hier steil ins Meer ab.
(The coast drops steeply into the sea here.)
Summary
In short: Use das Meer as the general word for sea or ocean (e.g., for a vacation). Use der See for a lake. Use die See for the sea in a more vast, nautical, or poetic sense, like the high seas.