海水（かいすい、kaisuい）
- sea water

The Japanese word 海水海水 (かいすい - kaisui) means "seawater" or "ocean water". It refers to the water found in the ocean or sea, which is characterized by its high salinity (salt content). It's a fundamental term in various fields, including marine biology, environmental science, and everyday conversation when discussing the ocean. It represents the collective water body of the seas.

⚫︎ 海水海水は塩辛いです。 Translation: Seawater is salty. ⚫︎ 私は夏に海水浴海水浴が好きです。 Translation: I like swimming in the sea during summer. ⚫︎ 船は海水海水の上を航海します。 Translation: Ships sail on the seawater.

This word is used in a wide range of contexts. You would use 海水海水 when talking about: ⚫︎ The composition of seawater (e.g., salinity, temperature). ⚫︎ Marine life and their habitat. ⚫︎ Recreational activities at the beach. ⚫︎ Environmental issues related to the ocean. ⚫︎ The source of various products, like sea salt. It's appropriate in both formal and informal conversations.

海水海水 is typically written with the kanji 海 (かい - umi, sea) and 水 (すい - mizu, water). The hiragana reading is かいすい (kaisui). There are no significant variations in writing.

⚫︎ 海水 (かいすい) itself is the most common synonym. ⚫︎ 海の水 (うみの　みず) - "water of the sea" (more descriptive)

There are no direct antonyms as such, but related terms might include: ⚫︎ 淡水 (たんすい - tansui) - "fresh water" (water that does not have high salinity.)

その魚はきれいな海水海水に生息しています。 ・Translation: That fish lives in clean seawater. ・Explanation: This sentence describes the habitat of a fish, using 海水海水 to specify the environment.

海水海水の温度は季節によって変わります。 ・Translation: The temperature of the seawater changes with the seasons. ・Explanation: This sentence explains that 海水海水 temperatures vary throughout the year.

The word 海水海水 is a compound word: ⚫︎ 海 (かい) - "sea, ocean" ⚫︎ 水 (すい) - "water" Therefore, it literally means "sea water" or "ocean water".