Water is a renewable resource. However, it takes considerable energy to treat water to make it potable (and eventually to treat in the sewer system so that it can be disposed of safely) so it makes no sense to waste it.

For showers there is the additional energy required to heat the water, although using a solar water heating system would reduce that.

One way to reduce the water wasted by showering is a navy shower. Essentially this involves running water only as much as absolutely necessary. I created business cards with instructions on how to take a navy shower - the PDF file necessary to print them is available here. You can use standard business card paper (I used a sheet with 10 cards - 2" by 3 1/2"). You could use plain card paper and cut it yourself. The PRF is designed to be printed double sided. I recommend laser printing - doesn't smear when wet. I distribute these cards at my local YMCA since many members think it is OK to use enormous quantities of water when showering.