In turning the Avett Brothers’ take on this gruesome true story into a musical, Logan and director Michael Mayer were facing a strangely paradoxical task. On the one hand, the events are there, the songs are there, and the central characters and situations are just waiting to be brought to three-dimensional life. Conversely, that very straightforwardness is a potential trap. A piece of theater assembled from preexisting songs, or charting a known historical event, can wind up feeling by-the-numbers: The things we expect to happen happen, and along the way folks sing about them (or, more likely, they sing about slightly generalized circumstances adjacent to the specific ones onstage). Despite its creative team’s efforts to lace a capital-T Theme through the work, Swept Away often falls prey to this roteness. Logan, with the support of Scott and Seth Avett and their bandmate Bob Crawford, has chosen “salvation and redemption” as the play’s big idea, but its deployment is telly not showy — we hear a lot about it, but our pulses never really rise with the stakes.