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.
Some publishers use the hyphenated phrasal adjective salt-water instead of saltwater, but the one-word form has been around a long time and is listed in most new dictionaries.
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]