Great topic!
About four years ago we were diving from a small charter boat off one of California's Channel Islands for a technical training class. The site was the wreck of a purse seiner type fishing boat sitting upright on the sandy sea floor at about 150 feet.
Conditions were excellent. When we reached the wreck we found it thickly covered by abandoned fishing nets. Trapped within the old nets was a three foot long leopard shark. A beautiful creature.
Two of the more experienced divers immediately took out their knives and started to cut the shark free. They gave up the dive they'd planned to help the poor animal. After about five minutes of heavy work, an opening was made in the nets, and the shark swam free. It was a thrilling sight, and one of the most heart-warming things I've ever seen underwater.
It's not very macho to say, but I still tear up a little when I think about it, even now.