First: gunboats never appealed to me. I've never played one, and don't intend to. As I wrote in my profile page, it's called "Diplomacy," not "Move a Bunch of Pieces Around the Board."
That said, I think there are several nice things about GB games:
1. They don't take a big time commitment. Because of this, you can play many at once without your life being sucked away by this site.
2. They require a different sort of communication—you have to review the move logs to see what happened sometimes, to see if, for example, a potential ally tried to convoy your army somewhere. Or, say you're Austria and start out with a hedgehog, but you notice that Venice supported Trieste's hold—good reason to believe Italy wants an alliance. This communication through orders is different than full press, but it's still a form of communication, and very interesting to gunboaters.
3. You get to know tactics very, very well.