I can only float (i.e. relax on my back doing nothing) in very salty seas, such as Aegean sea, and I absolutely cannot do it in fresh water. Despite it, my parents taught me to swim at 6, no way I'd be able to float before that.
The floating thing and even treading water are more or less required education before you're let out of the shallow pool/end and in to the part where you can't stand on your own.
I have seen adults that refuse to take lessons because... because... they are afraid.
So regardless I still say it is pretty crappy metaphor.