Use the pool exactly as you wish. You’ll find it’s worth setting aside the first few minutes of the session to actively focus on relaxing, i.e. assume a comfortable position, take a few deep breaths and concentrate on your body. Is your arm relaxed? How does your neck feel? Go over your entire body (repeatedly) and really let go. Now you’re ready to float! If you become aware of certain areas of your body during floating, or you’re aware of them for longer than usual, pay special attention to them in particular. Lie still. Don’t try to calm your thoughts. If your head refuses to "switch off", then leave it be and just ignore it. Let your thoughts wander as they will and simply concentrate on the fact that you are.
It doesn’t matter if you fall asleep during floating. You won’t topple over or go under. So floating is great for non-swimmers too! As you float, you can quite deliberately approach and explore the borderline between sleep and waking consciousness. And don’t worry if you swallow a little brine, it’s harmless.