Buckets are cheap, and with care and effort can be used to water various areas of the garden.
Points to consider are:
Limited amount of water
Most people only use 1 or 2 buckets in the shower. This will capture up to 4 gallons per person per day (not enough for the whole garden – but enough for prized plants).
Note a 10 minute shower (the shower would need to run this long to fill the buckets), running at 3 gallons per minute is 30 gallons of water, so 26 gallons of water is used to save 4 gallons.
The average person generates between 25 and 40 gallons of graywater per day in the shower, bath and laundry. So buckets are a great start, but only re-use about 10% of the graywater going down the drain.
During water rationing, people often concentrate on a limited number of prized plants. Always check the soil moisture level before adding extra water. Killing plants by over watering is a common problem in drought affected areas.
Apart from back problems caused by frequent carrying of buckets in the house and in the garden, beware of spilled water / slipping dangers.