Looking at California's total water usage [1] I would guess we are somewhere around 40 MAF per year (I hope. That can't be per day right??). Based on the probably incorrect numbers above, that translates into around a 20 GW demand for water. The diablo canyon nuclear power plant outputs 16 TWh per year. Terrible napkin math puts that at around 12 power plants needed.

Oof.

[1] https://www.ppic.org/publication/water-use-in-california/