海水

1. Origins, structure, and basic meaning

• Structure: 海水（かいすい） combines 海 and 水 and means seawater.
• Basic usage: 海水が道路まで飛んできた describes seawater blown as far as a road; くぼみに残った海水 refers to seawater remaining in a hollow in the rock.

2. Common pitfalls and similar expressions

• The place versus the water itself
  • 海 refers to the sea as a place or expanse, while 海水 refers to the water itself. 海水 fits the examples in which water is blown onto a road or remains in a rock pool.
• Contrasting kinds of water
  • 淡水 means water with very little salt.