Yeah if they had actually rang the ships bell, that would be really bad.
Well, all joking aside, it would be really bad as the ship's bell on a submarine is located inside the submarine.
These protestors only managed to set foot on the sub's outer hull.
Well and that was my thought also but the writer failed to be clear on that point. The way the statement was phrased seemed to indicate that the bell had some significance here, when the real issue is that these yahoos were able to breach security at a nuclear sub base.
When I was in the Navy I had one occasion to visit the nuclear sub base, but was told point blankly that you try to set foot on one of those boats without proper authorization, and you could very well get shot. But then again that was 30 years ago, I guess times have changed.