"one might consider a combo of a used scuba breathing regulator (Stage I&II) "
I would never trust used equipment
, of unknown origin and condition, to remain reliable for critical emergency
use. I've also seen "perfectly good" regulators freeflow after being in storage
for a few years, as the pressure setting invisible inside the first stage changed with age. Also wouldn't trust a used tank of any size, without getting at least a new VIP for it. And then, since most will also need a hydro...you're close enough to the cost of new to just buy new and *know* it is reliable. (Unless you're patient and knowledgeable and hit a bargain, including shipping
4 liter, 6 liter...No really. Wouldn't you want something more like a 19-30cf tank, so you could duck back under and salvage
the abandon ship bag? Or some food
and fresh water
? Anything like that?
And don't forget, the hatch
and any escape aids should be marked or painted luminous.