I don't see why arbitrary thresholds like depth, time or the observation of marine life should keep a dive out of your log book. People log for all different reasons, but the idea is to keep track of your diving. For example, if I were to get in the water with a new or rebuilt reg, and it were to have an unstoppable free flow at 5 feet and I thumbed dive after 10 seconds, I would absolutely want to remember that and keep that information with all of my other diving data. I have had very brief dives where there was a problem that I had to solve, and that's relevant to me.
Your log is your own personal record of diving. If you are going to "fake" it by doing 20 shallow bounce dives in a day so that you can qualify for some course or something, who is that hurting other than yourself? And if someone actually did that, how would it be different from just entering any number of fake entries in the log?
I have never encountered any situation where someone (dive operator or instructor) wanted to actually see my logbook. I'm not saying it never happened, but I just don't think it's as big an issue as it is implied to new divers.