There are large wildlife preservation efforts in the Bay Area marshlands (particularly near shoreline park) that would be destroyed.
Either way global warming is going to change the landscape. Might as well attempt something that limits damage
There was an absurd plan to place a dam between Richmond and San Rafael[1]. When it was finally studied, there were two fatal problems with the plan. (1) There is not enough water in Summer, to keep the bay behind the dam full. (2) There is too much water from snowmelt in early spring and the San Joaquin valley would turn into a lake.
Now to the environmental side: the bay is amazingly productive. It also acts as a nursery for the ocean. The damage done to the Pacific Ocean ecosystem would be all out of proportion to the gains of a little bit of land.