It's only a problem if you massively piss off the neighbor who supplies water, to the point that they wouldn't even sell it to you.

An independent California would just buy the things that it needs from its neighbors, like most other countries do.

Secession itself tends to massively piss of the country from which one has seceded, which points to why this would be a problem for an independent California (unless it was a “Greater California” including the headwaters and course of the Colorado River, etc.)

In many cases, you see the opposite - a strong public sentiment to allow or even force secession of some territory that is perceived as a "burden". This is usually represented in economic terms - "why do we have to feed X?" - but it can also be motivated by culture, religion, ideology etc (but, quite often, still coached in economic terms).