Deep wreck and cave diving is similarly dangerous (and was much more so before the adoption of Trimix by the technical diving community). There have been numerous fatalities among rescuers trying to recover corpses of other dead divers, too. I decided after reading a fair bit about this that even if I can afford to dive like this ($20k+ for equipment, $10-20k+ for training, and $500+ per dive for helium-based gas fil…
Have you read Blind Descent? One of the things that most struck me was that cave diving is more dangerous, fatality-rate-wise, than space exploration.
We know how to do diving in worse environments than caves relatively safely -- commercial and naval (Naval diving is working on docks and repairs for the Navy in peacetime vs. military diving, which is the kind of dangerous combat/commando/infiltration/specops missions done by SEALs and UDTs, etc.) diving to 500m is actually relatively safe as a career. This often takes place in HAZMAT environments, zero visibility, etc.
The issue is that recreational/"technical" divers are doing it with much less equipment, and vastly less support staff. A commercial diver has a $3-5mm recompression chamber waiting above, a safety diver ready to splash in, dive medics or medical officers, a dedicated support ship, unlimited surface-supplied gas, heated water in the cold, wired communications, etc. Recreational divers have what they can carry and personally afford to buy.