Popular Mechanics has a piece on California's concerns about rising ocean levels. Especially at risk are the cities around San Francisco Bay. Hat tip to RealClearScience for the link.
There is nothing technically difficult about building an earth fill dam across the Golden Gate and installing locks to pass ships in and out. Add some big pumps to keep runoff from overfilling the bay and a rise of several feet need be no particular threat to the greater Bay Area and inland low-lying areas like Sacramento and Stockton.
Actually, in this scenario a good argument can be made for sending the surplus water to SoCal via aqueducts instead of pumping it into the Pacific. SoCal can always use more fresh water.
Over time the Bay would become brackish and perhaps eventually fresh water. This would destroy the salt pan operators near Union City.