Couple of other thoughts:

BART: Marin's not part of the BART District. So to get service, they would either have to buy into the District (elections, politix, LOTS of money to pay for previous BART work elsewhere before new local work starts), or would have to pay for all or most of the work beyond SF (which *is* in the BART District) as Santa Clara Co. is doing. There have been occasional rumblings of discontent in Santa Clara about the cost ($billion$) and its effect on their ability to do other things of interest locally; and that's in a place where the travel demand arguably justifies BART. I wouldn't worry about BART entering Marin from SF.

Larkspur: if a full-scale rail connection is too expensive and unacceptable to the locals, and a moving walkway is to, well, dinky, how about a short monorail or linear-induction (not cable) powered peoplemover with a stop at the shopping center. Maybe even a maglev! Might be difficult to have enough capacity to transfer a SMART trainload to/from the ferry quickly, but it could be cute and therefore perhaps more acceptable to the locals.

SMART to Southern Marin: while also unlikely to happen in any of our lifetimes, it makes more sense than BART for local transit and could (theoretically, but at huge cost) be done. Much of the old NWP r/w is still there, though it's a bike lane in many places that would trigger vocal and possibly violent opposition. The Alto tunnel is still (mostly) there, though for SMART use it would have to be fairly comprehensively rebuilt. One problem is that large bits of the line south of Larkspur are very close to sea level - an issue with rising oceans (even now, don't use the lower parts of the Manzanita park & ride when Spring or King Tides are due) that could be temporarily dealt with, at a cost.