Not as big a barrier as several (tens of several?) feet of solid rock and no air between it and the water.
I think there SHOULD be a big barrier to going into a cave, and 1/3's on singles "these days" (i.e. with modern gear, training and knowledge) is pretty nuts.
It's not necessarily even the 1/3s (which is bad enough) but a huge single failure-point on the regs, plus the entire attitude of "There shouldn't be a barrier, it should be easy"
I understood the only single failure was the tank itself, as the valve must be "Y" or "H" and two regs.
of course, I suppose it's (supposed) to be a "free country" here in the US so who would I be to try to stop someone ?
If it is that important that people have a fully redundant setup, then why isn't anyone campaigning to change the requirements?
I think we have come full circle again.