I would say that the most dangerous thing I've ever done is to get a open water diving license abroad when I even couldn't swim.
Pushed by my friends, we four people went to Thailand for diving course, the course was quite speeded-up. We needed to finish all theory classes and swimming pool testing just in one day, and from the second day we were "thrown" into the real sea for practice... It may be not that challenging for a skilled swimmer, but I was kind of scared to death...Because I couldn't swim then, and also thought too simple before I realized that one coach needed to teach two students at the same time...
Everyday I struggled with whether to give up or not, because I thought blind boldness was not a good thing to stick to, but at the same time I wanted to make it out of my comfort zone...
Finally I got the license and I felt excited, and I learned to swim after I came back home.