Just to play alittle armchair investigator...
A lot of unanswered questions. Seems like there is more to this story.
A lot of possible explanations, disagreement about who's gonna steer? Wrestling match ensues, boat veers into rocks?
Is there assumed liability in being either a declared or defacto designated driver? If so, that would seem to put a damper on anyone's enthusiasm to be one.
Good mystery movie story line there somewhere