I have just done a BOSIET course to spend a week offshore. This included "Re-breather" training. A bag which most north sea passengers wear around their necks that allows them to re-breath their own air while affording time to escape an upside down helicopter in the sea. We did a dunker drill and as I sat there strapped into the up-side-down aircraft under water, I thought to myself... "this is nice.... Hummm...there is the exit....hummm...best get out I suppose!"
The underwater breath hold for an Gold medal swimmer in 10 dec C water was not over 1 minute, BUT JUST 15 SECONDS. Such is due to thermal shock and hyperventilation. To rebreath your own air helps to stop this and would give you about 3 minutes (or more) in which to escape, no matter how fast you breathed.
The bag is large when unfolded but only slightly inflates so you are not hindered in your egress and even if you were, you have lots of time to unhook any loose ends.
I am 100% for stass or re-breathers, but re-breathers seem to have the edge beacause you are not limmited by the speed you breath or a pressure indication error on the stass bottle.
(S76 nth. sea)