I do the same as described by holli4pirating and jockeys above. I used to use two cereal bowls one inside the other. Filling the larger one with the hot tap water. Later on I went for the small
Moss Scuttle and I love the thing now. My lather is warm, never hot but that is to my liking. I have some high end brushes and I wouldn't want to risk damaging them with too much heat. If the water is hotter than I can comfortably touch my face with then it is too hot for my brush. YMMV.