Just 3 miles west of Inverness is an excellent spot to launch a stand up paddleboard. The location is Drakes Estero, next to the Drakes Bay Oyster Farm. Drakes Estero is an unspoiled part of Marin County. It is a relatively shallow estuary teaming with wildlife - birds, harbor seals and sea nettles (these are not plants, but large brown frilly jellyfish).
The Estero is about 4 miles long from put in to the ocean. It also has several arms that are fun to explore - Home, Creamery and Barnes Bays. If the timing is right, it is possible to make it over to Limantour Estero near the ocean.
Timing is everything. If the tide is too low, the shallowness of the water could be a problem. If you are paddling near the mouth during a strong ebb tide, you may be whisked out to sea. If you want to try paddling all the way to Limantour, I suggest picking a time when the differential between low and high tide is not that large, and launch when tide is coming in. On a SUP the 4 miles trip takes about an hour at a leisurely pace. As long as the tide is still coming in, heading over to Limantour shouldn't be a problem. Just make sure you are heading back away from the mouth when the tide starts going out.
Equipment: Tide tables. The location you'll want is Point Reyes.
An excellent spot for launching that stand up paddleboard or kayak is Chicken Ranch Beach in Inverness. Like so many things in West Marin, there are no signs. Drive north through Inverness, and just as the road veers off to the west to Drakes Beach/Lighthouse/Tomales Point, you'll see some space for off road parking. That is parking for Chicken Ranch Beach. Portage your kayak or SUP board for about 100 feet to the beach. It's an easy beach for launching, even at low tide, and it won't do havoc to the bottom of your SUP board.