This may be a useful reference:
<Mixed Gases in Diving
Note that argon is apparently not a good choice. The hydrogen-oxygen mixture is interesting, if a bit creepy (BOOM!). I dimly remember a cheesy TV movie called "Goliath Awaits" which had a bunch of people who had lived in a pressurized section of a sunken ship for years, breathing an oxygen/hydrogen mix at that depth and living off hydroponics. The idea in the movie was that at that high pressure, very little oxygen was required in the mix, so it wasn't unusually explosive. I didn't buy it then, but this article says pretty much the same thing. Who knew?