It depends on the area, difficulty level and water temperature. Life jackets have pros and cons. Can be an obstacle when swimming after a drifting kayak or even worse rescuing somebody that fell in the water. A jacket will also block you to get impulse to return to the kayak in some cases (because you are unable to submerge). I often don't wear one but the children must wear it.
Of course in the sea the idea of not wearing a jacket is out of the discussion
This is exactly my point. Without knowing how common {wearing helmets in cars, wearing PFDs in kayaks} is, the statements imply nothing about safety benefits of either, and yet that one was presented in a way that suggests that it does.
What would be a non-misleading value to quote would be absolute change of risk of drowning (e.g. that a typical trip without a PFD costs X micromorts, while a typical trip with one Y micromorts). What would be less problematic (but still potentially misleading[1]) would be a relative risk reduction (wearing PFD makes you X times less likely to die).
[1] Because it doesn't tell you anything about the original risk that you're lowering. E.g. helmets are a good protection against falling bricks: equally good at construction sites as when you're walking down the street. The only difference is that falling bricks are more likely on a construction site.
While it's shoddy statistics, at least in this case the implied effect seems to be real.
According to these two sources [1][2] that just came up when I searched for this, prevalence of wearing life-jackets among kayakers appears to be ~78%.
Note though that getting base rates from one place and getting rates of accidents from another is usually a bad idea: they will have various ways of measuring the rates, so will have different biases (based on location, time-of-day, time-of-year, ... where the measurement happens).
