I have not made up my mind on PFDs (or seat belts) being mandatory. But I do religiously wear both (not at the same time).
Taking bicycle helmets as an example, doesn't seem so cut and dry:
- bicycle helmets protect a head from some hits to the head, but the vast majority of crashes it does not protect the person from. For example, if a bike is hit by a car, there will be so much trauma to the body that the helmet does nothing. And even if you endo, only if your head hits straight on does it really protect (not much protection from side hits).
- bicycle helmets have been shown to actually attract cars. Cars give less space to people wearing helmets when passing, but more space to women (so maybe we should put wigs over our helmets?): http://www.scientificamerican.com/article.cfm?id=strange-but-true-helmets-attract-cars-to-cyclists
- bicycle helmets scare beginners. If they think the sport is so dangerous that they need to wear a helmet, then they have a second thought about doing it.
The first and third have equivalents in kayaking. PFDs keep someone afloat, but my understanding is that the main cause of death (at least in the cold waters where I paddle) is hypothermia, not drowning (once the cold gets to them and they stop being able to swim, they do drown, but the real cause was the cold). Having a PFD would keep them afloat longer and make it easier to find the body. But maybe we should have required a wet suit rather than PFD - would give both flotation and warmth?
But the number of deaths in kayaking is very small, as compared to the number of people who do it. So does requiring a PFD scare people from taking up the sport? Quite possibly.