The San Francisco Bay Area is known for its very left wing politics and its booming tech industry. Both of these things have contributed to comically expensive housing.
The Bay is pretty shallow in most places and could easily be reclaimed, something that has been done numerous times, with SFO and the Marina district lying on land reclaimed from the bay. In 1959, the Army Corps of Engineers estimated that
325 square miles could be filled in, this land consisting of marshes, tidal plains, and shallow sea, turning the bay into basically a river. Any intention of doing this was ended by the environmentalist movement of the 60s.
100 square miles would have come from San Pablo bay, which would have disappeared entirely, as can be seen in the map above, and another 60 square miles could come from Suisin Bay to the Northeast. The remainder, lying in the bay proper, would've been impressive. Berkeley could've doubled in size. San Mateo and Hayward would've had just a river separating them instead of a giant bay. It would be possible to drive from Sausalito to Tiburon over the now-filled Richardson Bay. And all this land could've fit a lot of people. With a typical suburban population density of 6,000 per square mile, that's nearly 1 million extra people.
And if BART had been extended into Marin County as originally planned, the now filled San Pablo bay probably would've become host to a major urban area, easily accessible from Richmond. With the current population of the Bay Area standing at 7.1 million people, this extra land could easily have fit an extra 1.6 million, a more than 20% increase.
It would've been bad for the environment, but would it be much worse than the Dutch turning the salty Zuiderzee into the fresh IJsselmeer and then reclaiming big chunks of it? Or the staggering reclamation done in Boston, Manhattan, Hong Kong, or Singapore?
There is of course seismic risk, but that can be fixed by mandating tougher building codes.