Ive tried that several years ago ( before being interested by Rebreather
) by simply beathing in my BCD in a pool, using it like a pendulum Rebreather and flushing from time to time to eliminate CO2.
I think the greatest danger in shallow water is hypoxia.
Could be interesting to try that again, simply adding a p port and PO2 meter.
Why not add a small scrubber and some O2 too it? Unless you enjoy that heavy panting and anguish laden sensation of breathing a high CO2 mix ;-)