An idea I had a few years ago that I still find interesting, is that water is a product of the biosphere, from archaic life using chemosynthesis, 12H2S + 6CO2 → C6H12O6 (sugar molecule) + 6H2O + 12S. Could also not be that way and be abiotic, but, I liked the idea, that planets "sweat" water when they grow life. Maybe I am biased though towards a bio-centric world view, nature is just so beautiful.