The word for "sea" in French is "la mer". It is a feminine noun and it should be preceded by "la" (the) when used in a sentence.
Examples:
- J'aime passer mes vacances à la mer. (I love spending my holidays by the sea.)
- La mer est bleue. (The sea is blue.)
- La mer est agitée aujourd'hui. (The sea is rough today.)
It is also worth noting that in French, "ocean" is translated as "océan".