I can’t find good references to directly support the common claim that 80% of California water is used by agriculture. But I could find numbers on 2010 consumption. As shown below, irrigation accounts for 60.7% of water usage (this includes more than agriculture). It seems 80% figure comes from subtracting out water reserved for environmental uses.

2010 Figures:
mgpd = million gallons per day
Total = 38 billion gallons per day = 13,870 billion gallons per year
Irrigation (crop, pasture, recreational lands such as parks, golf courses) 60.7% (23,056 mgpd)
Thermoelectric power generation (equipment cooling) 17.4% (6,601 mgpd)
Public supply (domestic, commercial, industrial, pools, parks, firefighting) 16.6% (6,307 mgpd)

From California Wikipedia article
California GDP (2012) = $1.959 trillion
California agriculture production = $46.7 billion (2013) (~2.4% of California economic output).
California population (2014) = 38.8 Million

Upshot of this is ~2.4% of California economic output consumes more than half of California water.

I’m having trouble finding water use by agriculture commodity, but it’s easy to find agriculture economic output by commodity.
Milk $6.9B
Grapes $4.4B
Almonds $4.3B
Nursery plants $3.5B
Cattle, Calves $3.2B
Strawberries $1.9B
Lettuce $1.4B
Walnuts $1.3B
Hay $1.2B
Tomatoes $1.17B

Interesting blog on California water