To use your example, salt can be removed from water by boiling the water and condensing the resultant steam. This produces two products, distilled water and a dry salt.
The reason why this is possible is because salt is dissolved in water. Simplifying things, lets only consider common table salt, sodium chloride (NaCl). This applies to all salt. When sodium chloride dissolves in water it does not chemically react with water (H2O) to form hydrogen chloride, HCl, which is hydrochloric acid, or oxidized sodium (NaO). The salt stays in solution and when water is removed salt remains.
You mention tritium, which is one of the isotopes of hydrogen. What defines an element (hydrogen, oxygen, carbon, iron, etc.) is the number of protons in its nucleus. Hydrogen has one proton, carbon has six protons, oxygen has eight.
The other particle that an atomic nucleus can contain is a neutron. Protons have positive charge, neutrons have no electrical charge. Electrons have negative electrical charge, they orbit the nucleus.
Commonly, the nucleus of a hydrogen atom only contains one proton. However, sometimes it can contain one or two neutrons. This makes such atoms of hydrogen heavier than a typical atom with only one proton. A hydrogen atom with one proton is called deuterium and one with two protons is called tritium. When such atoms combine with oxygen to produce a water molecule the atoms are locked together.
Boiling such water only produces steam. If you want to separate deuterium and tritium from water, the water molecule must be split to produce hydrogen gas and oxygen gas. This can be done using electrolysis, where an electric current is passed through water.
Regarding the other radioactive wastes in the water, such a strontium and cesium etc. These elements can be removed from the water as you suggest, by boiling the water. This produces another problem, what to do with the radioactive power that remains when the water has been removed? How much energy must be used to do this and what does it cost? What form of energy will be used to do this: nuclear, renewables, coal, gas, solar evaporation? How much radioactive power would be produced and how is it then collected for "disposal" and how and where is it then disposed of?
Dumping radioactive water into the ocean is cheap and rids the utility of a inconvenient water storage problem. It gives the utility an opportunity to "move on" after so many long tortuous and expensive years.
