I don't really know much about SF or local utilities. I know agriculture takes up most of California's water supply. But even though a relatively small amount is consumed directly, would there be enough water to substantially house 2x-3x the current population? How much would price spike?

Well, a lot of the people who can't afford to live in SF, but would like to, still live in California, consuming California water. They just live farther out in the north, east, or south bays.

Of course, if SF actually had 3, 5, or 10 times the population it does today then there would probably have to be a lot of infrastructure changes too. But the limit probably wouldn't be available water for residents.

Household water usage, especially for dense housing like apartments that lack yards, is such a small percentage of overall water usage that it's a rounding error. Agriculture and then industry are the main users of water.