Something’s off here. Basic math says that adding one more tenant to split the bill should make each share smaller, not bigger. The fact that the opposite happened is worth investigating.
$130 per unit in an 8-unit building means roughly $1,000/month total water. That’s high for most California buildings - not impossible, but unusual enough that I’d want to see the actual bills.
Here’s what I’d do: send an email (so you have a paper trail) asking for copies of the actual water company bills for the past several months. You’re not being difficult - you’re just trying to understand what you’re paying for. If the landlord refuses or keeps dodging the question, that tells you something.
You can also call the water utility directly. Most California water companies will give you historical billing info for the address. It’s a quick way to verify whether the numbers your landlord is giving you match what the water company actually charged.
If you hit a wall and still suspect you’re being overcharged, your local housing authority handles these complaints. The landlord’s allowed to add a small admin fee, but they can’t turn the water bill into a profit center.