Having read the thread so far I think everone is agreed that the wearing of LJ/ PFD is generally a good / sensible thing in most circumstances and should be encouraged.
I would not dream of being on a small craft without an LJ/PFD, but if on a yacht it mght be that being clipped on with a harness so one doesn't go over in the first place is the "safer" option (OK so you could have LJ and safety line).
Legislating for this will be very difficult in terms of definitions, exceptions etc (e.g. if I'm swimming off the boat, at what point do I have put an LJ back on?). Knee-jerk, needless or over simplifed legislation is rarely good law.
Who would enforce it and how?
I wouldn't want more delegated legal authority to harbour authorities or rescue services to levey fines turning them more into another police enforcement (revenue raising) organisation. There would be nothing to stop a person removing a LJ out of a harbour limit in any case. I wouldn't want marine police lurking around evey headland waiting to pounce on "random" inspections.
I would suggest that if life is lost at sea / on the water needlessly (or to be more accurately - where it could have practicably been avoided) by an individual wearing a life jacket then it should be possible to prosecute the captain of the vessel for failing in their duty of care towards passengers under current legislation - be it friends / family or paying passengers, if it's in the public interest.
If you take this further for leisure users, what else might be legislated for - minimum safety equipment every boat must carry,compulsory VHF fixed / handheld on every craft with random inspections and fines levied by harbour patrols? Every boat could have to be coded - there's an industry waiting to happen - a compulsory appropriate licence for every boat helm / captain etc...... Each of these COULD possibly save a life. Where would you draw the line?
Is education not better and let people take their own decisions / responsibilities?