Can water be too hot for yeast?
Yes. Water that's too hot will kill yeast. Most types of yeast begin to die around 130°F. Active dry yeast should not be put in water above its ideal range of 105-115°F, and most recipes that use instant yeast call for water that's 120-130°F. The less common fresh yeast does best at 95-100°F.
Yeast is a living organism, and can't survive in water that is too hot. At 120º Fahrenheit (48º Celsius) it will start to die off. At 140º Fahrenheit (60º Celsius) all the yeast cells will be dead. The ideal temperature for proofing yeast for baking is 95-100º Fahrenheit—similar to body temperature.