We’ve discussed the use of moisture sensors for showers before, and one of the problems is that once the contacts are wet, they take awhile to dry out. So it all comes down to the details of these cases. They can work for showers in some households, but if you have two people, or one person who for whatever reason ends up taking multiple showers sometimes, it’s possible that only the first shower of the day would be detected.
It seems to me that’s even more of an issue for a kitchen sink, where the sink may be used more often than the moisture sensor is able to detect.
Some people have gone for humidity sensors for shower detection, which have the advantage that the humidity drops back down fairly quickly. There’s one pretty sophisticated method in the following topic:
And some people have gone for the vibration sensor on the water pipes. @jmay33 has done some work on this for his medical smart home project.