You mean the "environmental impact" of lowering the sea level in the Pacific
The environmental impact of any desal plant itself is that it dramatically raises the salinity of the water near it's outflow, the water is not lost from the normal hydrological cycle. You can minimise the salinity problem by not placing your outflow in a shallow bay. Wind, wave, and tidal power are ideal for desal plants since they are normally built near the coast, those built in deserts can obviously use solar. Unfortunately the one they built here in Melbourne was accompanied by a new brown coal plant which will only accelerate the unwelcome feedback loop between the climate and our species.
Desalination from seawater costs about 8.5 kWH / m^2. That is a lot of power.
I think you mean cubic meters, not square meters.
waste heat from existing power plants via secondary heat exchangers
Usable heat is already converted to electricity, that's the one thing a coal plant does best.