This third boat scene [Mk 8:14-21] is climactic in emphasizing the incomprehension of the disciples. By citing the feeding miracles and addressing such matters as lack of perception, hardened hearts, and the failure to remember, Jesus' words recapitulate the substance of the three boat scenes and the two feeding miracles. Despite auspicious beginnings, the disciples, by the end of this series of scenes and miracles, show themselves to be like "outsiders." Like "outsiders," they "think the things not of God, but of humans" and regard reality from a this-worldly point of view. Of course, unlike "outsiders," the disciples follow Jesus and are "with him" in commitment to his cause. Accordingly, incomprehension on the one hand and commitment on the other are hallmark traits of the disciples. Jesus' struggle with them is to lead them to overcome their incomprehension lest it undermine their commitment to him. This is the central issue in Jesus' relationship with the disciples, and this third boat scene highlights it vividly.
--- Jack Dean Kingsbury, Conflict in Mark, 101-102