In Marin, for public transport, there's bus and ferry to San Francisco. For Bart (regional metro service), one would have to go over the Richmond bridge or the Golden Gate bridge (and far enough into SF that you'd be downtown already). Getting to Bart stations via either bridge is really quick as long as you have a car. Bus and ferry are quick enough. Also, you can bicycle to the ferry or even across the Golden Gate from Marin.
In Marin, for public transport, there's bus and ferry to San Francisco. For Bart (regional metro service), one would have to go over the Richmond bridge or the Golden Gate bridge (and far enough into SF that you'd be downtown already). Getting to Bart stations via either bridge is really quick as long as you have a car. Bus and ferry are quick enough. Also, you can bicycle to the ferry or even across the Golden Gate from Marin.