Can Billie go alone, or did he say he needs someone to go with him?

The previous sentence, "[The boats are] also pretty small," serves as the justification when Billie says, "I will only go with Devin alone." Obviously, what Billie is saying is that the maximum number of people a boat can carry is limited. As a result, Billie needs to be selective with whom he chooses to go with, and so declares that he would only go with Devin. It also stands to reason that since Billie's preference is influenced by capacity concerns, he should not have any issue with going alone. There is nothing in the context of the problem suggesting that Billie would not move unless Devin is in the same boat with him, so it is logical to assume that your latter interpretation is indeed correct.

In short, for logic puzzles, we should not assume from more than what we are given. We are given that the boats are small - forcing Billie to limit the people he goes with - but we are not given any conditions that could justify Billie not being able to cross alone.