It requires helium mixes and personally I'd only want to do it with a rebreather, not normal "blowing-bubbles" type SCUBA gear (too wasteful on the breathing gas). 20-30 minutes at the bottom will mean around 90-120 minutes decompressing. You don't want to be doing anything taking too long, or being too laborious.
Could be done from a fishing boat without any problem.
Having had equipment failures no deeper than 30m, I decided any dives that require decompression are not for me. I'll leave deep dives and sabotage to others :)
It would seem simple, being able to deep dive then without decompressing. I'm sure there's a good reason it's not being done, I just wonder what.
You can't breathe pure oxygen under water, as it becomes very toxic very quickly as pressure goes up. So you have to dilute it with something. Nitrogen is easy to get hold of (actually you just compress atmospheric air). I'm also not sure cutting nitrogen out helps much with decompression sickness, you get it with other gases too - maybe more slowly, I'm not sure.
You can, and do, replace nitrogen with other gases, mostly helium (and in rare circumstances, hydrogen), but for other reasons. Nitrogen too becomes toxic under pressure, around the 30-40m mark, with symptoms similar to drunkenness - not good under water.
Funnily enough, helium does the same deeper down, except the symptoms there are the opposite - nervousness, tension etc. So to some extent perhaps these two balance out. Hydrogen has very low toxicity, but for some reason people feel queasy about mixing pure hydrogen and oxygen.
Crucially, too, a lot of deep diving is commercial in nature, and cost-sensitive. Anything that isn't just compressed and bottled air is getting very expensive very quickly.
It all gets incredibly complicated. Deep enough, you need to breathe gas that contains so little oxygen that you would suffocate on it at surface pressure. At pressure, it's fine - the partial pressure of oxygen is sufficient, and having less of it makes it less toxic. But you have to be careful to only use this gas deep enough, else you'll run out of oxygen. Or at other times, when decompressing, you might use an oxygen-rich mix, which while more toxic, helps you purge some of the other gases from your bloodstream, cutting decompression times.
It's a fascinating topic to read about from the comfort of my sofa.
Also, you would need to have pure oxygen available and breathe that for maybe an hour before each dive, which would be expensive and annoying. For touristic dives, especially along the shore, it's easier to just plan the dive so you can still see something interesting while waiting for decompression.