Because oxygen and especially hydrogen are such a pain to store compared to water some rocket designs contemplate just leaving all the fuel mass as water and only splitting it into its parts as needed before firing.

Water is also effective at blocking radiation [0][1], making it especially useful when stored around the spacecraft occupants.

[0] https://www.quora.com/Can-we-use-water-or-ice-as-radiation-s...

[1] https://www.nasa.gov/pdf/284275main_Radiation_HS_Mod3.pdf (page 4)

Don't you need an energy source to turn the water into its parts? That's the energy you recoup when you burn the hydrogen. Or did I miss the joke?

Presumably this would be done with a small nuclear reactor powering an electrolysis system, in which case it’s feasible. Your savings are in complexity and weight because containing a mass of water is trivial, and naturally dense compared to H or O. As an extra feature you need water anyway, it can be used as a coolant, and you need oxygen which you’re now generating as a byproduct. Presumably for cruising you’d use the nuclear reactor to power an ion drive and only use the Water splitter as needed for maneuvering, launches, and landing.

South pole - perfect place for constant solar power. Just raise it up a little bit.