That had indeed crossed Levi's mind, but it was for that reason that he agreed to wait for the others in the first place. Bowing out of the task had never occurred to him. After all--

"I promised to get her to safety," he says, as if that's an indisputable fact that explains everything.

And sure, he'd done that before he knew there would be any water travel involved, but he wasn't going to go back on his word just because of that.