Can water be too hot for a silicone sex doll?

Yes. Silicone tolerates heat far better than TPE, but that does not make boiling water safe. Very hot water can dull or damage the finished skin surface, and the more immediate risk is to you — water hot enough to scald is dangerous to handle while you are supporting a heavy doll in a tub or shower. Use hand-hot water, the same temperature you would wash your own hands in. If it is uncomfortable to hold your hand under, it is too hot.