Nubian,
as you will know then, inflating a life jacket under water inside the helo is prohibited in huet trng for obvious reasons. If you are unconscious under water, correct, you will drown.
However, I was attempting to respond to the scenario described by nigelh.
Landing a helo in shallow water could very easily result in the aircraft spilling the occupants out or allowing them to escape relatively quickly thereafter (rotors stopped). For those with LJ's on that jacket will prevent them from drowning if they are disorientated or can't swim. Without LJ's what do u reckon their chances are?
That, I would suggest is why the authorities (UK) require LJ's outside of autorange from the coast for pax carrying helo's.