Can you use the oxygen trick to practically extend snorkelling dives?
I used to do a little scuba, but overall didn't like the reliance on often poorly maintained kit. But I do love snorkelling - the lightness and simplicity of it.
Can I breathe pure oxygen for half an hour on the boat and be able to repeatedly snorkel longer?
The desire for breathing comes from CO2, not oxygen. Pre-breathing pure oxygen will likely not have a significant impact on non-trained free divers
Pure oxygen also has its own issues and risks which is part of the training for people who use it. It is not a stretch to call it the opposite of simplicity.
Even though you can do it with pure oxygen, it won't be fun. This is because our breathing reflex activated from the rising carbon dioxide levels - meaning that for a significant portion of the attempt the person was fighting their urge to breathe.
shallow water practice is the best way to gain capacity and confidence, or some sort of winhoff method....sitting on the couch or whatever as it is a simmilar flex that you can drop into any sort of free STATIONARY time, ie not while driving, or on top of anything like that, or before doing a dive, even just blowing your head up, blowing baloons, again within limits, as I believe that there are possible ways to injur yourself doing that, but as an easy to do (very rough) capacity test that can be repeated as often as desired for almost no cost. I got to the point where I could swim a full lap(two lengths)in an olympic pool underwater with no trouble, and only a few extra breaths before going, it seemed to be mostly up to following a very moderate and steady stroke.