You asked two questions there.
Watching a thread.
I think there is something like that, which is called 'Tracking' a post. I think it happens when you scroll down to read a post automatically, or a user can click to do that. When you track a thread, you get notifications within the website itself (next to your username icon on the top right corner)
I believe it is upto each user if they want to enable that. The default is to send weekly digests and not send individual notifications, but it can be modified by each user in their user preferences.
Do play around and let me know if that works.