The deep end is the same water as the shallow end. The only thing that changed is how much you trust yourself.

Water does not become more dangerous as the floor drops away. The physics is unchanged; what changes is that you can no longer stand, so the whole thing rests on believing you will float. The fear is about the absence of a backup rather than about the water itself.

Build the trust in the shallow end where the stakes are low. Float on your back until it is boring, then do it somewhere slightly deeper. Most confidence is made this way, by proving a small thing repeatedly before you need it.