I don't buy that one. What powers does a lifeguard have when someone just says "bog off, i'll swim where I like".
Consider a scenario where the swimmers are ignorant of the existence of the ‘danger zone’, which the lifeguard is responsible for monitoring and protecting people from, and the lifeguard fails to take any action to dissuade them from the zone.
^That would have elements of negligence and responsibility that may result in manslaughter.
If however a lifeguard has done all he could reasonably be expected to do, then as you say it wouldn’t be an appropriate charge.