Last week we were diving in Grand Cayman.
The first dive of the day was a max depth of around 100'.
We headed over to the second dive site, which was a very short boat ride. Some of us were jumped off the boat, to cool off, float around, snorkel, (pee!

), whatever, during our surface intervals.
Upon hearing that the top of the coral-head below was only about 40', this one guy tells his wife that he's going to dive in and see if he could touch it. He then proceeded to try to do so.
Bear in mind, that we were only about 15 minutes into our SI's!.
The DM was quite irate and told the guy that "I've seen people get bent pulling stunts like that! Do not ever do that again!"
Fortunately, the guy did not seem to suffer any injuries, but that sure was an eye-opener and a pretty valuable lesson.
I'm also kinda embarrassed to say that as I watched the guy pull that stunt the thought that it could be dangerous never even crossed my mind!
I've definitely filed that in my "lessons learned" category.