Suppose that SF became 20 meters under sea-level (!), and was protected by a giant sea wall. The city's area is about 122 square km, so the total volume of water displaced by it would be around 2.5e12 liters. The ocean's surface area now is about 362 million square km, so (if I'm calculating this right) the displaced water from SF can be expected to raise the water level by about 7 micrometers. I'm not sure I see the problem here?