Seawater is a liquid which is mainly used to get Water and Brine. It can also be used to get rid of Carbon Dioxide, by combining it with Limestone in a Gas Injection Pump. Seawater can also be directly turned into Salt in an Evaporation Pond.
In the early game only the Basic Distiller is available. However, in the mid/end game, the Thermal Desalinator becomes available. Considering both buildings one will be more efficient then the other. When comparing, the total cost is used, thus for the Thermal Desalinator, the Boiler (Coal) costs are also taken into consideration. Also the recipe with the Steam Hi is used, since this is the easiest to setup. In terms of coal, the Basic Distiller uses 6 Coal per 60 Seawater, the Thermal Desalinator uses 4,5 Coal per 60 Seawater. The net gain of Water in the Basic Distiller is 36 Water, for the Thermal Desalinator the net gain of Water is also 36. It produces 48 Water, but also consumes 12 Water for the production of Steam Hi. Also in terms of Brine both buiding produce the same amount. In the end when comparing both buildings, the Thermal Desalinator is more effecient, because it uses less coal.
When Brine is the desired output, the Evaporation Pond is the best building, since it produces 32 Brine per 60 Seawater. When Salt is desired the recipes chosen don't really matter for the cost of sea water. It all depends on the desired setup of buildings.