Most Californians do not have pools. I live in an area where they are very rare --
Anyways, when you are under rationing, it is your choice how you use your water. You could not flush the toilet but once a day,let the lawn die, not garden, for example, and top off your pool.
There are pool covers that float on top -- kind of like bubble wrap -- that cut way down on evaporation, and other expensive child safe covers that work even better.
And, as per usual, if you are rich, you can just buy more -- either by going over your allotment and paying the fines or by trucking in water. I here some rich households in expensive areas of southern California are paying to bring in water that way, so it is not thru the city water mains and is then not counted. I am very unclear as to where the water trucks are getting the water. Proabably the same way the bottled water companies get water, there seems to be some kind of work around for businesses in the water bussiness