I've read on this very thread that it is due to the increased temperature of the 'exhaust' water, it can only be so hot and exhausted into the esturaries.
That's one of the reasons. Warm discharged cooling water apparently isn't what the environmentalists want.
The cooling water that is being used runs through the condensor (think of a big radiator or heat exchanger with the enclosed steam/water being cooled by outside water from the stream) for the purpose of cooling the steam back into water after its useful energy has been expended on the turbine. This condenses the steam into water for 2 purposes - one, to be pumped back into the boiler, and two, when the steam is condensed into water, it creates a vacuum that increases the efficiency of the turbine/generator by pulling the steam through with even more vigor than it would have had. When the cooling water is warm, it does not cool as much, and thus condenses less steam creating less vacuum and less energy output.