none of these are realistic or reasonable
We could have carried more oxygen
what size of tank? how many extra tanks? how many more rescuers would have been needed to carry those tanks? do you crack and put a reg on each one prior, or do you have 1 reg and move it between tanks as they run out? this is a recommendation from someone who has never carried multiple tanks into a scene, or had to change a reg on scene. Too much equipment weighs you down and frankly just gets in the way.
could have transported him out of the system faster
faster than what? faster then the team that was moving as fast as they could to save his life? did you guys stop for tea and crumpets on the way out? This wasn't an episode of CSI, you guys did what you could, as efficiently as you could, with the situation you were presented with. Rescues are dynamic, period.
could have provided oxygen whilst we transported him out of the situation (but this would have killed him even faster as it was all below 6m)
so he died because you didn't give him O2 in a situation that would have killed him
faster if you gave him O2? so the inquest wanted you to kill him faster then? It sound like this inquest doesn't know their ass from a hole in the ground when it comes to underwater rescue at depth.
if we hadnt been exhausted (700m sprint swim) we could have probably carried on CPR for a bit longer
what an ignorant comment!!!!!!!!! what exactly is "a bit longer" (2 minutes? 10? 60?) exactly how many more minutes would have definitively brought the patient back? this is wishful thinking, not based on fact and knowledge of human physiology. And what are you supposed to be, super human? I have to point out the obvious, Superman and Batman weren't on this rescue, mere mortals were, and the human body fatigues, period. There's a reason why the compressor changes out after 2 minutes
max. when doing cpr. Also, if the patient is asystole, they're done and you can do cpr until the next millennia, but you're not going to change that. Ditto for brain death.
notable f*ck ups like being late
this is the only finding that you mentioned in your post that might have any merit, but it's context absent, so it's hard to say. You can't save the patient if you aren't on scene, but why aren't you on scene? There's a difference between dragging one's ass and being late, versus having something legitimately interfere with one's ability to get on scene.
From your post, it sounds like you and your team worked a difficult rescue, in the best way you could. The patient died because he was already dead, not because of any f*ck ups on your part. Are there some things to be learned to make the next rescue more efficient? sure, because EVERY rescue is an opportunity for learning, but that doesn't ever write a guarantee for the next rescue. People die, it's just a fact.