Not an authoritative source , just a blogger, but a decent attempt to explain some of the cali water issues that's reasonably well sourced itself.
California receives a total of 80 million acre-feet [99 km³] of water per year. Of those, 23 million [28 km³] are stuck in wild rivers (the hydrological phenomenon, not the theme park). These aren’t dammed and don’t have aqueducts to them so they can’t be used for other things.
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14 million acre-feet [17 km³] are potentially usable, but deliberately diverted to environmental or recreational causes. These include 7.2 million [8.9 km³] for “recreational rivers”, apparently ones that people like to boat down, 1.6 million [2.0 km³] to preserve wetlands, and 5.6 million [5.9 km³] to preserve the Sacramento River Delta. According to environmentalists, this Sacramento River Delta water is non-negotiable, because if we stopped sending fresh water there the entire Sacramento River delta would turn salty
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It's possible that the claim is based on something like the view pushed by the wall street journal
The Wall Street Journal says that farms are a scapegoat for the water crisis, because in fact the real culprits are environmentalists. They say that “A common claim is that agriculture consumes about 80% of ‘developed’ water supply, yet this excludes the half swiped off the top for environmental purposes.” But environmentalism only swipes half if you count among that half all of the wild rivers in the state – that is, every drop of water not collected, put in an aqueduct, and used to irrigate something is a “concession” to environmentalists. A more realistic figure for environmental causes is the 14 million acre-feet [17 km³] marked “Other Environmental” on the map above, and even that includes concessions to recreational boaters
Conclusion: Sorta.
It may depend on what you consider diverted to mean, if you only count when you divert water out of a river and into an aqueduct then release it somewhere else like into another river or wetlands that eventually sees it enter the ocean then this only seems to happen to a relatively small amount of water.
If you count water that you could easily divert at the press of a button to other uses like agriculture but instead allow to remain in the rivers flowing into the ocean then a larger fraction would count.
If you use a very broad version where you count overarching political choices about where to build dams and aqueducts and choices about economy vs ecosystem (which I believe would be an unreasonably broad interpreation) then you might count most fresh water that you allow to flow into the ocean.
Even the strict interpretation where you only count actual physical diversions include a few percent of cali's water which is a lot of water in absolute terms or relative to cali urban use.
