Garry, if you can't get behind the shower wall for access, there is a problem.* You can cut around and go in from the control side of the valve as long as the new cover plate covers the hole.* This is really tricky to do as the valve assembly can be larger* than the hole.* To change it, you may have to cut an access at the rear.*
Since most boat water heaters are small and set at a high temperature, a temperature balanced valve is to be disired.* It can keep you from getting scalded.
Be sure to seal tightly around the cover plate.* Water getting behind it can cause allot of damage.