So here's what most likely happened to this guy, since most people who aren't divers may be confused as to what happened to him.
He's diving at a place called the Blue Hole in Dahab, Egypt. It's a natural sinkhole, but is unique in the fact that it connects to the open see through a natural "arch." It's notorious for killing inexperienced divers that try to swim through the arch and either can't find it, underestimate its depth and run out of gas, or get nitrogen narcosis from the wrong mixture and eventually drown. He did not die trying to find the arch and you can read about the countless arch deaths elsewhere.
He most likely was doing a "bounce dive" to the bottom, which is where you just plummet to the bottom and come up immediately, usually to break a personal depth record. It is incredibly foolish and dangerous. You can see him show his dive computer to the camera multiple times, so it's probably to verify to people that he actually hit the bottom. He was diving with a SINGLE tank of AIR. This is the major contributing factor to his death. At 90 meters (10 ATM), he only had 1/10th of the gas in that tank available because of the pressure. He also was using AIR which is 78% nitrogen. At depths below 100feet, NITROGEN becomes intoxicating. This is called nitrogen narcosis. At this depth it probably felt like he downed 8 martinis. Also, OXYGEN is TOXIC at great depths, and results in seizures and ultimately death. So while you MAY survive the nitrogen narcosis at great depths on air, at depths greater than 190 feet(56m) you are increasing the chances you are going to take an oxygen "hit" and convulse and die on your next breath. And this is only two of the many ways you can die scuba diving :\ That is why technical divers that are diving deep use mixes of gas called trimix that replace some of the nitrogen and oxygen with helium, which is inert, so they can keep a clear head and not worry about oxygen toxicity.
When you dive, you need to balance your buoyancy with your BCD, which you inflate with gas as you descend. Once again, if he filled it up all the way at sea level, at 10ATM (90m) it would only have 1/10 of the volume. That's why you have to keep filling it as you descend, which is the hissing noise you hear. It was discovered that he was also overweighted with heavy camera equipment. Overweighting is common with new divers (they were not shown how to properly calculate the amount of lead weights to use) and causes them to constantly have to fill/dump air in their BCD and their buoyancy goes to **** (It's called "yo-yoing")
Okay so let's put all of these mistakes together.
1) He was diving AIR, which should never be used below 190 feet (~58m) because of the oxygen toxicity, and is rarely used below 130ft (~39m) anyways because using trimix will prevent the nitrogen narcosis so you can actually remember your dive. Yuri was in lala land at 90 meters for sure.
2)He had a single tank. At those depths you might as well just learn to freedive really deep and just hold your breath
3) He was overweighted, which caused him to have to empty his tank into his BCD when attempting to ascend.
4) He was diving alone. I don't think I need to explain.
At the end he probably almost emptied his tank trying to inflate his BCD to ascend. When he wasn't ascending, his breathing rate would naturally rise, causing more of the toxic mixture into his body. You can see that he most likely goes into convulsions from an oxygen "hit" at the very end. This would cause the regulator to fall out of his mouth. Nitrogen narcosis will actually lower your seizure threshold. There was unlikely enough gas in his tank anyways to get him positively buoyant. He did not get caught in the sand (whatever that means), attacked by a shark, or try to yell for help like someone suggests in the video. Just inexperience, poor planning, and frankly, stupidity. Also, There is a video on youtube of his body being recovered by an experienced technical diver. When you see the equipment and preparation it takes to go to 92 meters safely, you can appreciate the dangers that accompany deep diving.
TL;DR: A combination of nitrogen narcosis, oxygen toxicity, a single tank, improper gas mix, and overweighting killed him. Not a shark, not the bottom turning into quicksand, not a zombie diver. Just inexperience, stupidity, and probably arrogance.
