In any real world plant that operates via evaporation, most of the energy used to boil off the water is re captured via the heat exchangers used to condense the steam back into water. This heats the incoming water. (Most are multi stage running different parts at different temperatures/pressures.) Overall its a pretty efficient process, with reverse osmosis a bit more efficient.

However the amount of water that we use for not drinking purposes, irrigation, washing e.t.c is massive.