Those who claim “they can’t swim” are trying to keep the water at CHEST LEVEL. This is impossible. They fight it until their arms tire and then they start slipping under.
This is a very interesting observation. I will use this with my grandchildren.
I picked it up by noticing exactly where I floated if I remained perfectly still, and held my breath. One can float motionless if one breathes shallowly. You might have them try holding their breath for about 10 seconds and floating relaxed. It helps to know you don’t have to constantly fight to stay afloat. The more relaxed you are, the easier it is to swim. Like a frog or a fish.