In most parts of California, or the US for that matter, there is no water to filter if the water system were to break down.
And if the issue was viral contamination of water, nearly all portable/emergency filters will not have an effect since viruses are so small.
But we're not looking at the water system breaking down. If that's a concern now, why wasn't it equally a concern six months ago. Why not have water hoarding back then? Similarly, we're not looking at scenarios with viral contamination of the water supply. Why throw crazy what-if scenarios on top of the actual problems we need to deal with?
The issue at hand isn't "the world is coming to an end", it's "everyone needs to go home and hang out _only_ in their own homes for a while - not just if you feel like it or it's convenient. We've got a lot of people who are acting like we're living in post apocalyptic Mad Max times, trying to gather a ten year supply of bottled water and toilet paper and canned food "from before the bombs went off", and that's not helping.
Florida had beaches full of people playing "Spring Break", as if the rules didn't apply to them, up until just recently. That finally all(?) got shut down, and now those idiots are going home, so they can kill grandma with the virus they picked up at the beach. We need to reboot the world, and we need
everyone's cooperation regardless of whether or not it's convenient, and we need people to stop running what-if scenarios from post-apocalpytic movies through their heads, because their resulting actions are hurting other people.