If you install a bar thermostatic shower, be aware that this type don't withstand prolonged freezing conditions. I have had two where the hot side inlet valve has stuck in the closed position. Once I managed to fix it with gentle persuasion; on the next occasion, (winter 20120/11), my powers of persuasion broke it for good.
That said, the one I have at the moment was fine over this winter, with a little heater on during the cold spells. You may remove it as part of your winterisation process if you don't live aboard.
To save water you may want to try installing a Venturi shower head handset. I have heard mixed reports, so depends on your pump setup I suspect. Where they work well, water consumption is considerably reduced.
Good luck.
Bob.