I don't know if this will be enough to make me want to replay the whole game. If there are additional cutscenes that could enhance the story, I feel like I'd like to see them as the game unfolds.
A boat doesn't really excite me either. Maybe if turning the Regalia into a plane would be less of deathtrap, I'd be intrigued, but not really. If anything, the Regalia should have been made to become both a boat and a plane.

There really wasn't all that much motivation for me to finish the extra dungeons, I don't know if more of them would help