Hi,
Of all that, I wonder why you want to use an out put only device as an input device? If you are going to ' Force ' , as you said, water into your urethra, the pressure could seriously affect the sphincter muscle of the bladder as well as some fine blood vessels of the urethra, it is only my expectation, you will find out when you try it. In any case, please let us know as to why you are planning to do it?
Solace
I wonder if anybody with normal health has tried to instill water through urethra. However there is a yoga technique which asks to suck water through your pens, keeping it in water. By breathing in and creating a vacuum, one can withdraw water through penis.This is called vajroli kriya. You may google the word vajrolikriya to satisfy your curiosity or try it if you are interested in yoga.