You can also use a large plastic tub if you do not have a large enough sink. Adding ice can cool everything down too. Though you will have to keep adding it and watch the temp to maintain a consistent temp. You conversely use an aquarium heater to warm the bath if it is too cold ambient.
One thing to point out additionally is that if you use a plastic tub and it is sitting on a cold concrete basement floor that will lower the temp as well.
I have found comparing the temperature of the water bath to the temp of the fermenter (stick on thermometer) it is usually about 2 degrees difference actually. Likewise with a fermentation chamber. For instance, I have 2 6 gallon carboys of bitter fermenting right now. One is in the chamber (with a heating pad) the other in a water bath with heater. in both cases the carboy is reading 2 points lower that ambient (air in the chamber, water in the bath) when the most active fermentation was going on at the beginning it was opposite.