It has proved to be absolutely watertight.
Personally I hang it around my neck, I have tried it on my arm too with success. Don't forget to orientate like a nurses watch, upside down.
The control unit would cost so much to replace. I think it only sensible to keep it away from dirt and water from the off.
I would buy again, and look upon it as a serviceable item, that will need replacing now and again.