Thanks for the comments and no offence taken , it all make sense to me, but think I'll stick with my recreational diving for now. Plenty to see and do between the surface and 130-ish.

Interesting thoughts re: redundancy, the obvious things like sufficient air (gas), backup computer, plan on a slate etc. however, things like a spare mask would probably not have been on my list but as soon as you mention it, it makes complete sense. Presumably, all stuff one would learn to think about during the appropriate tech training.