I just have 10 gallons of water, plus a water filter -- I live about 200 feet from a freshwater lake, so I'll have access to water.
Actually, I have 2 water filters, the primary one is a LifeStraw Family that claims to be a purifier that can filter both bacteria and viruses and will last for 5000 gallons), and I have a small hiking filter (a katadyn, I think) and some purifying tablets as well.
I also have a "WaterBob" bathtub water bladder so if there's a disaster where I still have water and think I might lose it (like if there's an earthquake+power failure, I might have water for a few hours), I can fill it and store 60 - 80 gallons of water.
Oh, and like you, I have a tank style water heater so have 40 gallons (150 liters) of water in my hot water heater that will probably be salvageable unless the earthquake knocks it loose from its straps or there's a water contamination issue (which the filter will probably work for). In that case, the house may not be livable either, so I'll probably be in a shelter anyway.
This was all pretty cheap, excluding the hiking filter (which I owned anyway and cost over $100), I paid around $80 for everything.
I just looked online and the Lifestraw is selling for $170, I paid less than a third of that price, might be a lot of people buying them now so inventory is scarce.