The Japanese word for sea is
海

うみ - umi

Japanese Definition

海
1. n. a sea, an ocean
(antonyms, ja, 陸, tr=oka, riku)
海の中にはいろんな生き物が暮らしています。 - There are all sorts of creatures living in the sea.
海に行こうよ！ - Let's go to the beach!
2. n. a lake
(synonyms, ja, 湖, tr=mizuumi)
3. n. (figurative) an object covering over a large area, as in 血の海 or 火の海
4. n. (planetology) a mare (dark surface of a celestial body, thought to appear like a sea)
5. n. (calligraphy) in a 硯, a reservoir where water is stored
(synonyms, ja, 池, tr=ike)
6. n. Usage: This is the most common general term in modern Japanese for sea or ocean.
7. prefix. of the sea, of the ocean
8. n. (obsolete) sea, ocean
9. affix. sea; ocean