Salt is dissolved homogenously in water that is why
Salts are in general totally dissolved in water. That is why we have "total dissolved solids" (TDS) parameter in water quality engineering/environmental engineering.
If more than soluble salt is in the water, it precipitates (more than maximum soluble amount).
However, the evaporation temperature (vaporization temperature) of salts are higher than 100 degrees Celcius. At 105 degrees Celcius, water vaporizes but salts stay.
It consists of only one physical phase.
The designation of homogeneous or heterogeneous as applied to mixtures depends on how many distinct phases are present. In the case of something like clay dispersed in water, there are two distinct phases - solid clay and liquid water. In the case of oil and water dispersion, the two liquid phases are not miscible, so can only form a dispersion of small droplets in one another.
Salt dissolved in water however, undergoes a change in chemical structure whereby the salt lattice breaks up into separate sodium and chloride ions that become solvated by water molecules. Therefore there is only one physical phase; distinctions are only apparent on the atomic/ionic scale.
Salt solution is therefore a homogenous mixture, unless of course it is past saturation, so that not all of the salt can be dissolved. Then it would be heterogenous.