I don't mind Moll and L'ak being couriers. It makes sense, plot-wise, for couriers to be digging around on an old ship like that.
What does bother me is Moll being Book's kinda-sorta sister or something. For one thing, it is absolutely not needed. We don't need a repeat of "Oh my God, my enemy is someone I care about deeply" from season 4 (Which we're still dealing with the ramifications of, to boot) Secondly, it brings us to a preposterous level of "small universe syndrome". Like, the bigger the universe gets, the smaller it becomes? WHaaaat?