This has nothing to do with screenwriting craft ... but here's some fascinating footage of a tiger shark apparently about to take a chomp out of a woman swimmer when a dolphin apparently wards him off. (Dolphins are nice to people, but they also have a habit of battering sharks to death, which is probably why the shark chooses discretion over valor.)
Dolphins are also famous for pushing drowning people towards shore.
And I thank them for that, but ... why? Not like we're all that nice to them, is it?
Not that I'm a marine biologist or anything, but it looks like the shark approached the diver and realized it wasn't food, and then left. Sharks rarely attack humans and usually it's because they have mistaken them for something else. Usually they take one bite, realize they have made a mistake and let go. There's actually lots of video I've seen of sharks taking a run at a swimmer in the water and then changing course at the last second when they realize it isn't their food.
Dolphins are cool, though. Maybe they don't talk amongst themselves, so they don't know that their relatives are being slaughtered by the thousands by humans. Or perhaps they don't make the connection.
I think dolphins surely identify with us in some way. They seem to know we're mammals, and (in the water) we're swimming mammals about their size. It looks to me that the swimmer was facing the pod of dolphins, so maybe they were relating to each other in some way. So the dolphin had the natural impulse to ward off a shark who was getting too interested in an honorary member of the pod.