Marin is the smallest of the NorCal counties, and it's not exactly known for its surf. It is all beachbreak with rare exceptions. This is also one of the sharkiest places on Earth (it's the northern boundary of the so-called 'Red Triangle'), so Marin is hardly worth visiting if you're not feeling lucky. Most of the county's shoreline is within the immense Point Reyes National Seashore, a wonderland of tangled woods, serene estuaries, grassy knolls, and windblown (and wind-sheltered) beachbreak. But, if not for the surf, check this place out just for its unspoiled vastness.
When the rest of the Marin coast is flat, there will be some kind of surf at Point Reyes Beach (north side), albeit often junky. It's a good option during the summer if the winds are calm. The place sticks so far out into the sea, it's susceptible to all kinds of weird currents and wind fronts coming from the open ocean. The waves do get good in Marin, albeit seldom, and since distances are far between surf spots, you'll surely encounter some sluggish tourist traffic, especially on weekends. Bolinas is a hot area, as is Stinson Beach. North, at the mouth of shark-infested Tomales Bay, Dillon Beach sees a fair number of surfers on any given day, but Bolinas and Cronkhite Beach are by far the most crowded (Cronkhite is most easily accessible from the San Francisco Bay Area). Check out other beaches along the Golden Gate National Recreation Area and Marin Headland for other good spots that come and go as conditions vary.
Point Reyes blocks winter NW swells from making it into Bolinas Bay, but a W will get in nicely – a good place during bouts of huge winter swell and NNW winds, which blow offshore at the south-facing beaches. Bolinas Bay does pull in S swells, although they are rare and when they do arrive, the place is packed with surfers. The south side of Point Reyes is also exposed to S swells, which take the prevailing summer NW winds right in the face. Of course, S winds spell disaster at these spots.