Depending on the kind of yeast used, water temperatures will vary. Red Star Platinum Yeast necessitates a slightly warmer water temperature, about 120F to 130F. If using other brands, follow the manufacturer's recommendations, but 100F to 110F is typical. I know some people just dip their finger into water and if that's the method you're using, err on the side of warm bath water rather than hot because you don't want to risk killing the yeast. In bread-making, I don't like to guess and always use my candy thermometer. I just never use it for candy!