If you are drinking 15 cups of tea in your working time, then unless you have a catheter fitted, it would seem safe to assume that you are having to go to the little girls room quite often as well.
I don't know what your job is but if you are spending all day drinking and peeing, I would have thought your Boss is entitled to query this.
I am not sure what the issue is here ? Yours Boss is paying you for a job of work, so isn't he allowed to look after his investment ?
Incidentally, why ARE you drinking so much ? Have you had a diabetes check from your GP ? I found myself drinking increasingly huge amounts of fluids a few years ago and subsiquently found that I had diabetes.