I'm not a tai-chi expert but I spent 4 years doing Aikido - it was an interesting art and taught me some useful knowledge, but as a self defence art on its own it would potentially do more harm than good giving practitioners a sense of false confidence. There are far more effective arts that can provide simple and effective techniques at an early stage and then build on them. Perhaps the most effective art is british or thai boxing - simply because it teaches speed, evasion and fitness. It's amazing how tired and disorientated you get when you're in a proper scrap. None of the Aikidoists I've ever trained with would last 2 minutes if it came to the crunch, even the high degree black belts. I did however enjoy it, hence 4 years of doing it. It's a beautiful meditational art when you master a technique.
Yes, you're right on water. It's the most important thing. If we don't drink we're in trouble within a week. I guess because I'm in the UK there is no shortage of rain.
There are a number of options to purify and clean dirty water. Bits of charcoal, coffee filters, tea towels and kitchen towel can help to filter it, a drop or two of bleach can kill bacteria. The plastic bottle in the sun technique would work better in a warmer climate.
I have a couple of 20L barrels of water that I just refresh every couple of years which would get me through a week or two if I only used it for drinking. It might be a bit stale but it would get me to the point where I could source more.