just thought id post our experience since it was less than a month ago.
you'll be fine getting a ticket at the harbor. i'd recommend ging a day or two in advance to see how fast its filling, and the camping at Playa Tecolote overnight (or a few days) before your departure. awesome beach we had all to ourselves just 15 minutes out of lapaz.
unless its a sunday, then expect a bit of company :friday:
we opted out of baja ferries because we have our dog, but took the cargo ferry across and loved it
http://www.ferrytmc.com/
maybe a bit less plush, but you can sleep in you car if you want and it was a lot more of the "local" experience (and saved money to boot).
we try to go a few days in advance to book tickets and the just laughed and had us come back 2 hours before departure. 4 other couples have all followed in our footsteps since our ferry trip ad all has gone well (although a few reports say you should plan more time than the 2 hours..i would still check in a day or two prior).
heres the full report if it holds interest for you.
http://www.thedangerz.com/cargo-ferry-to-the-mainland/