The person learning to float is practicing a kind of trust their body may resist.

Floating asks you to stop doing the thing your body most wants to do, which is to grip and keep your head clear. The instruction is to release and the reflex is the opposite, so the difficulty is not strength but a disagreement between what you know and what you feel. Being told the water will hold you does nothing to quieten the reflex. Trust of this kind gets built by repetition rather than by argument.

Wherever you are trying to let go of something, expect early attempts to be undone by reflex and count them anyway. Short repeated goes teach the body more than one long determined effort.