Getting a bath or shower when the grid is down may not be possible without hot water, unless you are comfortable with jumping into a freezing cold bath! This article showcases a video explaining how to make hot water when the grid is down, and can be done by anyone who has the right tools.
What you need to get going is some copper piping, and a wood burning stove. You then coil up the copper piping, and place it inside the stove. As water runs through the stove, it heats up to several hundred degrees, and then is fed into a storage tank, from which it can be used.
The video below explains the process.