There is a box next to each watched thread, after checking the box(es) what can you do with that thread and How?
If you click on the tab on the far left of the header a drop down menu will show you options one of which is watched threads this will allow you to find any threads you are watching quickly. Also if you have notifications on it will let you know when any new posts have been added to any of your watched threads.
I was just trying to figure out how to unwatch multiple threads. I believe I have around 400 or so and I need to cull them once in awhile lol. I used to be able to check a box next to the thread I wanted to unwatch on the watched thread list, then down on the bottom right there was a small drop down box with options to decide what to do with the check marked threads. I could do multiple threads this way. I finally discovered how to do it the new way. Open your watched threads, go to the bottom and click show all unwatched threads. When you do that a small empty box will open right at the beginning of the title of the thread. Check mark all the ones you want to stop watching. Then down at the bottom of the list is a drop down box with all the things you can do. I only use it to stop watching threads. No idea if this has been a problem for other folks, or whether this was even what this thread was trying to get at and I just didn't get it lol.