My buddies Alex Fogg, Joe Kistel and I had been filming and collecting lionfish at various sites in the Gulf southwest of St. George Island FL. We had been taking turns with Alex and Joe diving together while I dove solo. This was my third and final dive of the day on a man-made structure in 93' feet of water approximately 22 miles due south of St. Vincent Island. The shark enters the camera's field of view at 3:48 and a moment later catches my eye. I instantly recognized it as great white and you can faintly hear me say "Oh my God". I estimate the fish was 35' or so below me. It spooked me in that it had approached from behind and had already passed me when I saw it. I watched until it swam out of sight then re-cocked my speargun which I usually unload when heading back to the boat as a safety precaution. I glanced around looking for the shark and up at the boat to make sure the dive ladder was indeed down. I noticed a small lionfish had escaped from my bag and was swimming down and you can hear me say "oh no". The bag of lionfish was positively buoyant and it drifted into me dragging the stringer of mangroves with it. I moved it out of the way and kept scanning the water column below. My first two dives had been to 120' and I had pushed this dive at 93' to my limit of bottom time. I needed to do this safety stop to avoid the bends, so I stayed where I was while my computer counted down the required 3 minutes. I took another quick look at my boat and another at my computer which shows me at 15' below the surface. Most sharks have a tendency to come back for another look and that's exactly what this one did. Fortunately, I was looking in the general direction when I faintly saw something that caught my eye. It was the shark about 40' away coming straight at me from below. I've had countless encounters with bulls, sandbars and a few tigers and hammerheads. Over the years, I've learned the best defense is to go on the offense and let them see that I'm not intimidated. Turning and running could trigger the predator/prey instinct, so I stood my ground aiming my speargun at its head. For a second I figured this was it and was prepared to fend it off with my gun like I have done countless times before with curious or aggressive sharks. Fortunately, for whatever reason, this one turned at about 15' away and gave me a full profile view. I say "Oh my God" again and then let out a loud sigh of relief when he turns again and heads back down. I watched until it disappeared again then decided to take my chances with decompression sickness rather than a third visit from the shark, so I skipped the remaining 2 minutes of wait time and headed to the surface and safety of the boat. I estimate the shark was 9-10' long and at no time did it seem aggressive or anything other than curious. I'm a charter captain and I had a juvenile great white around 12' long circle my boat last year about 30 miles out around the same time the tagged great white shark named Katherine was being tracked in the Northern Gulf. I had heard other reports of random sightings and saw the news reports of the great white caught off the beach in Panama City so I knew great whites come into the Gulf. This was the first one I've ever seen while diving and this experience will make me reconsider diving in murky water with limited visibility. I'm glad we both went our separate ways fully intact and healthy, but I hope I never have to experience another encounter like that again.