His GoPro camera that he was wearing captured the moment a huge great white swam over his head, and then appeared to circle him for the next minutes.
Jimmy can be seen keeping his front to the shark, holding his spear out to jab him in case he gets too close.
At certain points Jimmy does lash out with the spear as he sees the shark getting closer and closer.
Shark experts recommend that if you do encounter the fish when diving or swimming, you should not let it out of your sight until you are safely on shore or on the boat. They also recommend staying as still as possible - as Jimmy is seen doing in the video.
When sharks are swimming slowly and smoothly they are generally seen as unthreatening, but if they make sudden movements or start zig-zagging, it may be looking to attack.
Other shark safe tips are to avoid swimming at dawn and dusk - as this is when the hunt, and steering clear of areas where seals live, as they are their prey.