Why take showers at all? A large bath filled with warm water that could be sterilized regularly would be far more luxurious and last quite a long time.

Also pro tip: to maximize water you need a mister type shower head that sprays smaller droplets, which gets you far wetter and is more efficient than a regular rain type shower. Bladerunner 2049 featured such a shower and I think a longer running version of that for Antarctica would be fantastic.