I've never had to resort to this -- here in NH we have had a lot of rain this year, so the reservoirs are in OK shape -- but I have heard of using gray water for plants. Not having a gray water system, this could be done by allowing bathwater to cool and then scooping it up into a watering can or bucket to use on thirsty plants and shrubs. (If this is the plan, I'd avoid extra chemicals in the bath like bubble bath or oils, and just use soap, which usually contains phosphates which work as fertilizer, and are not harmful.)

Here are some other tips:
- plant natives, they are used to your conditions and don't need as much water once established
- consider letting your grass can go dormant in the summer, it'll come back in the fall (less mowing too!)
- mow at the highest setting your lawn mower will allow
- if you don't want to let your grass go dormant, it needs roughly one inch of water per week, use a rain gauge or tuna can to measure how much you are watering (when the tuna can is full of water, you have watered about an inch)
- set up your sprinkler so it doesn't water your driveway or other paved surfaces
- water in the morning. regular watering in the evening can provide mold and fungus growth. if you water in the middle of the day it'll evaporate more quickly.
- use a chopsticks to determine if the soil is wet - stick it in the ground for a few minutes, if the wood looks wet when you pull it out, you don't have to water
- pay attention to the weather forecast. no need to water today if you will get an inch of rain tonight
- use rain barrels to collect water, (but don't use that water on veggies because of the roof run-off)
- big containers need to be watered less often than smaller ones

My neighbors have lost multiple expensive tree & shrub plantings during heat waves or drought but my system's pretty foolproof and I've never lost a tree or shrub. I always plant with a 2" or 3" PVC pipe alongside, the pipe is situated to hit the root zone of the tree or shrub, I drill a few holes alongside so the water is dispersed at various levels in the soul. Then in drought or extreme heat in California I either fill that pipe with hose water or greywater to keep the trees & shrubs watered (minimizing wasted watering that only penetrates the top layer of soil with the remainder of the water evaporating). I take special care to use this method with any expensive plantings including all the fruit trees. The pipes aren't visible, I generally have something small planted around them disguising the pipe - and - I'm consistently one of the lowest consumers in my area, on water bills, yet my garden's very lush in comparison to my neighbors.

I take help of professional landscaping companies to fight drought. Gottashopit is the place for me from where I get connected to top-rated landscapers. I just post my job there and let the landscapers bid on my project. I do get amazing bids and I get my lawn care work done at best price from the experts!