Pompeii is accessible by train, but it's a long walk to the train station, and again, if you're not very comfortable in uncomfortable environments, then I would book a tour from the boat. It's easy, very easy to get lost in Naples if you get off at the wrong train station. I am sure that many people, including myself, have walked around Naples and have been fine, but even I have been approached for nefarious means and it's not fun. Just be safe and take a tour. That being said, you were with a large group of people and want to walk to a cafe someone, then you'd probably be okay. Naples has a high crime rate, and frankly, I have never met a warm and friendly Naples local.
I haven't written a blog post about Naples yet, but I will soon, with pictures, I hope this helps for now.