> try to get by on a two-minute shower once a day

This is pretty typical aboard ship for smaller vessels on long voyages. Not so hard as it sounds: get wet, turn off; lather up, rinse off, turn off. You can do with 20-30 seconds of water and be quite clean, with some practice.

It's not satisfying though. And I can't imagine how those numbers scale if you have long hair, or use conditioner.