Is it possible to watch dissertation defenses?

While they are open to the public, that usually means other professors and a few graduate students. They often don't even bother announcing them to the entire department, much less the general public. You'd have to contact departments and ask when they will be held in order to attend. It's very likely that no one will appreciate having someone in the room who doesn't have a college background in the subject; if you do go, don't ask questions. They are often on a time limit, and it's not their job to explain basics.