No - they're independent functions as far as I know. You 'watch' a thread for future ease of reference and you 'bookmark' to signify eg where you got to in reading it. I generally don't rely on Watching - I go with the changed colours for threads I've read but you can do the two separately. I don't think there's any separate list of threads that you decided to put a bookmark in. (Happy to stand corrected on that one, though.)
If you remember what thread it was that you put the bookmark in, when you click on it, you should go straight to the bookmark. If it doesn't, then maybe it didn't 'take' for some reason so you might have to start skim reading to remember.
cozie is right. The thread is still wherever you left it. Bookmarking merely allows you to remember how much of a long thread you've already read and when you re-open it, you'll be taken to that point instead of having to re-read.
You need to use 'watching' to file away a thread for a later date.