You might try planting native plants from your area. They're already
used to living on rainfall alone. I have a dry garden here in
California and I've had good luck with natives (from here) and
succulent plants. Succulents, like cacti, are great with drought,
except they cannot survive frost or snow. We are always trying to
recreate English gardens here, surely you have lots of wonderful
natives to plant. Also, try picking your battles; group high water
plants together, or pick one area that you want to 'save' with water in
existing plantings. Read up on the plants you are growing now, perhaps
their water requirements are lower than you think. You'll have to just
keep an eye on them while you both adjust to new watering schedules.

I feel your frustration with water rationing. We had it here a few
years ago. -And the farmers continued growing RICE in the high desert!
The polymer idea is good, but it is hard to implement on a large scale.
Mulching helps to keep the soil moist. Adding lots of organic matter
(like peat moss) also helps. If you have a compost pile, use your
compost as mulch - it works great and is also good nutrition for your
plants. Strong, healthy plants can resist drought longer than weak
plants.

Reusing household water is also great. In the US this water is called
"Greywater". You might try a search on 'greywater systems' that will
store and filter water for you. Plants can only take so much soap. I
usually just save my rinse water from the laundry to cut down on the
soap. I've used my soapy water on the lawn but I try to alternate with
'fresh' water. Perhaps the rain will help you there...
We have a soap here called Ivory, it is pure soap which breaks down
well. Look for 'soap' as opposed to 'detergent' on the label. Good
luck!