Not saying you can not. I am pointing out that you have to know what it is you’re using and how it works to be sure you’re understanding how to do it right. I see it done wrong all the time. And as stated; exact water heater temp is not (+/-) 5F critical.
A dirty kitchen sink has about the same emissivity as water anyway. If you run the water long enough, the sink conducts to the water temperature anyway.
You can not say you are measuring the sink through the water though, because you are not.
