The one-word saltwater is an adjective that describes salty bodies of water, the water of such bodies, and the life that lives in such bodies. The two-word salt water is correct in all other uses. It’s a noun phrase, with the adjective salt modifying the noun water. Similar distinctions apply to freshwater and fresh water.
The department says the saltwater solution is potentially better because it can be applied up to three days before a storm. [Boston Globe]
This program recognizes anglers for catching freshwater and saltwater fish of qualifying size. [Stamford Advocate]
After breakfast, I totter down to Chuan Spa’s 12-metre saltwater pool for a few laps. [Sydney Morning Herald]
Ms Savage has a desalinator on her boat to make salt water drinkable. [BBC News]
More than 40% of the farmland of the costal city of Sendai, for instance, has been soaked with salt water, sludge, cars and garbage. [Wall Street Journal]
We raised our heads out of the sea and removed our masks, salt water streaming down our faces. [The Province]