Do I need to be a member to become a watcher, or to leave comments, on Deviant-art?

You have to be a member to watch people on Deviantart (or you can add their profile to your Favourites bar if you want).

As for comments, this is straight from DA itself: Please be advised you must be logged in to post comments to users.

I strongly encourage you to join. It's great community to be a part of for all forms of art.

You cannot become a watcher on Deviant Art unless and until you have become a member; which is a very easy step and I advice you to do so. It will only take a few minutes.

But you can leave comments without being a part of Deviant Art; however it will appear only as anonymous. You will not be notified in case the artist replies to your comment.