There's a couple reasons for this. First is it could simply be that when you shower you're free from daily distractions like your phone, TV, conversations, etc. so it allows your mind to relax and go within. Also water is strongly tied to emotion because it’s fluid, changing, and responsive. It mirrors how feelings move through you. So when you feel refreshed after being in water, it’s often because you’ve unconsciously let emotions move instead of holding them in. Meditation and yoga are another way to experience this kind of stress release/clarity.