Old topic, but here's how I lost a fin temporarily several years ago.
Shore dive off Coral Street or Otter Cove (I forget which) in the Monterey area. They are similar sites. Both have a long shallow area to swim through before you get to diving depth ~ 5' deep and on that day there was a lot of kelp (the kelp has since given way to grass).
The kelp kept grabbing my legs, so occasionally I would reach back and pull the kelp away from me. Well one bit of kelp seemed to be REALLY strongly attached to me. So I pulled it with all of my strength and it finally gave way....
Except it wasn't kelp, it was my spring strap that I pulled. And I pulled it so hard that it lost its spring and my fin drifted away. And I didn't immediately notice. After a bit of funny swimming, I find that I only have one fin.
Luckily, my buddy was able to swim back and find the fin (viz was about 3' there) with the spring strap stretched out. He was impressed with my strength in being able to defeat a spring strap.
I had to return to shore where I had the original rubber strap, so not only did I not lose the fin, I didn't lose a dive.