The crossing takes 6 hours, and with about an hour to go we were outside looking at the stars and noticed a small boat powering over right towards the stern of the ferry. It pulled up alongside and a man wearing khakis and a golf shirt and no life jacket hopped off the little boat and climbed up a ladder onto the ferry. As the boat pulled away we saw "Piloto" printed on the side. It seems that there are a lot of big rocks to avoid when pulling in to Topolobampo.
There's a bar right in the middle of the ship, as well as really horrible Spanish subtitled movies on satellite-TV if you grow tired of the scenery outside.