Zeno

Depends on what you mean by a meaningful amount. According to recent finds by Curiosity (http://www.bbc.co.uk/news/science-environment-24287207), Martian soil is pretty rich in water: So 100,000 cubic meters yields, by my math, about 3342020 litres. Most missions could likely do with far less than that. Actually, in situ production based on Martian conditions has been tested pretty extensively in the lab. NASA has looked into it a lot. It's been demonstrated to be both viable and efficient.

This is essentially an Android port of olex's transfer orbit calculator at http://ksp.olex.biz/ The code On the Play Store All credit for the maths and such goes out to olex et al., I just basically ported it.