The principal problem with Larkspur Ferry parking is really that it has poor connections to other modes, especially bus. Though there used to be a shuttle system in place, it didn't do well and was cut years ago. While the Wave has taken a step toward reintroducing the shuttle, Golden Gate Transit has ignored regular bus service from the 29, as well as daytime and weekend trips to and from the ferry. To help riders get a visual of their options, I've created an integrated bus/ferry schedule (PDF) for routes 17, 25, 29, and 228 - all of which serve Larkspur Ferry Terminal at some time or another. The Interurban light rail schedule (PDF) did the same thing with the Sausalito Ferry.
On the weekdays, what stands out to me is the very long connections for people coming from San Francisco. Though the 29 does pretty well for those heading to the ferry during the day - most require waits of only 10-15 minutes - it's awful for connections from the ferry. Most connections are between 20-30 minutes, a couple leave only a minute to spare, and just a handful are in the sweet spot between 5 and 10 minutes. Optimizing the time points between the bus and ferry could boost ridership all on its own, without any need for new service.
Study the schedule yourself and you'll see what I mean. And, if you're a frequent ferry rider, print it out and keep it in your schedule book.