The deep end is the same water. You’re just more awake in it.

The water does not change at the line on the floor. What changes is that you cannot touch the bottom, so your attention comes fully online, and that alertness is the entire difference between the two ends of the pool. Being more awake is uncomfortable, and it is also the state in which people learn fastest.

Notice where you are staying in the shallow end out of habit rather than need. The easier class, the same role, the familiar route. You already know how to swim. What you are avoiding is the feeling of not touching the bottom.