The freshwater is one output
Desalination does not make salt vanish. In reverse-osmosis desalination, pressure drives water through a separating material while much of the dissolved salt remains behind. The Department of Energy describes the concentrated residual water as brine. Thermal desalination instead evaporates water and condenses the vapor; it also requires energy, even when some heat is recovered.
Sources: s1
An intentionally simplified balance
Imagine 1,000 kilograms of feed water containing 35 kilograms of salt. Suppose an idealized process produces 400 kilograms of completely salt-free product water, with no other losses. The remaining 600 kilograms must contain all 35 kilograms of salt. Its salt mass fraction is 35 divided by 600, or about 5.8 percent. The feed began at 3.5 percent.
This calculation is an illustration, not a plant specification. Real systems have pretreatment, imperfect salt rejection, operational losses and additional material streams. The example shows why reporting only freshwater output leaves an important part of the account unfinished.
Useful comparisons need system boundaries
USGS describes desalinated water serving municipal, agricultural and industrial uses. Those uses do not demand identical treatment or produce identical economics. When comparing technologies, we would ask what enters the plant, what product quality is required, where the concentrate goes and which energy inputs are counted. A claim of 'free water from the sea' should name its energy source and residual stream. Neither abundant seawater nor a successful demonstration establishes a complete, maintainable water supply.
Sources: s2