Using the water from tubs is virtually the same impact as the sink. I had two waterers in the winter, although we had electricity where they were and kept it reasonable for them. It was more out of our convenience in the cold. We carried a new waterer out in the winter full and mixed with vitamins, electrolytes and anti/pro biotics they don't get during the sub zero winter every day, trading out the one from the day before. Cleaning a sink takes no time.