Assuming your psychological state of mind and your heart health is relatively good, whole body cold water plunges/swimming 3 times a week for 2-3 minutes, has been shown (in relatively small studies) to have many positive effects on various endocrine system functions, including cortisol levels.
I don't know if you've seen the documentary 'My Octopus Teacher', but the subject spends an entire year diving in frigid water. His voice and general presence are so peaceful, it's almost comical. I joked at the time I saw it that he must have no cortisol left.
