I get the frustration. Paying the same as a family of four when you’re one person living alone feels obviously unfair - because it kind of is, even if it’s not technically illegal.
California doesn’t require landlords to split water any particular way. They can divide by unit, by number of people, by square footage, or use a formula called RUBS. What matters is that whatever method they use should be spelled out in your lease. Check what yours says.
If the lease doesn’t specify, or if it’s vague, you might have room to push back. Even if it does say per-unit, nothing stops you from asking to renegotiate at renewal time. “I’m a single senior paying the same as units with two or three people - can we work something out?” Landlords don’t always say yes, but reasonable ones sometimes do.
It also helps to ask for copies of the actual water bills. You want to verify the total makes sense before worrying about how it’s divided.
One more thing: if there are other single-occupant units in your building who feel the same way, there’s more negotiating power in numbers than going it alone.