It sounds to me like you're experiencing the wonderful effects of Earthing! Swimming in a natural body of salt water is THE most effective & fastest way to rid your body of toxic-frequencies AND uptake the natural negative electrons we're all starving for (from constantly being out of contact with the earth). If you felt that good after only 20 minutes, just imagine how healing it would be to Earth for 8 hours (while you sleep). Of course, you could have some 'mineral uptake' happening, as well...but your fast & healthy response is very typical of Earthing.
All of the great healers throughout the centuries had people 'barefooting' and/or making sure their bodies came into contact with the earth (or natural bodies of water) everyday. Many had their patients sleeping outdoors on the ground, and a couple even buried them up to the neck (lol). The frequencies of our Earth are just as imperative to our 'electrical body' as oxygen, water & nutrition (this is explained in the book, link below).
Earthing daily/nightly is known to bring cortisol levels into natural balance/rhythm along with a WORLD of other benefits (including reducing anxiety, and inducing a sound and restful sleep). Many people have a natural & safe increase in melatonin when Earthing.
Everyone with adrenal issues (or any health issues) should look into Earthing mats, sheets, etc. We've had some amazing testimonies over on my forum (here's a post with a compilation of some of the testimonies -- be sure to check 'em all out if you have the time). //www.curezone.org/forums/fm.asp?i=1838582#i