Update 1: 

I assembled the pipe and valve and tested using a bench power supply. The valve actuated and the system seemed to work as intended. No water left from the shower head while the valve was open. I tested at 12V and saw 0.323A (3.87W) power consumed.

 I need to create a waterproof enclosure for the electronics and solenoid and consider how to fix this to the shower wall.

 I also need to test the battery system for the following:

1. Power consumption while holding solenoid open (predicting 1.1A at 3.6V)
2. Approximate discharge curve/battery capacity (expecting well under 2300 mAh) 
3. Charge rate in various ambient lighting conditions