I answered this in this thread:
Basically the short version seems to be that sometimes Alexa doesn’t support everything yet. I am reading that Alexa doesn’t yet have built in support for leak sensors. I don’t know why, since as you said, it seems like on/off would be exactly the same. You should realistically be able to set them up as if they are a contact sensor. So I don’t understand why it’s not that simple. Maybe there is something in the use-agreement that prevents them from misrepresenting what kind of sensor it is just to have it supported on there.