San Francisco may have a harbor and bridges, but it's hard to drive right into the water. After 911 they added balusters to prevent vehicles from leaving the roads, so you would not be able to access the water near a bridge from a car. Just because you have water next to land does not mean you can actually drive off a road into the water. For that you need a boat ramp, and while there are a few (maybe 3-4) such ramps in San Francisco, you will find that using them is a lot less convenient than driving across a bridge and you will need to go far out of your way to do that.