Why do Katherine Hepburn and Cary Grant spend so much time talking about boats in The Philadelphia Story? I understand that the True Love, their yacht, is a metaphor for their relationship. What specific advantages does the boat metaphor have though?
It's a way to talk about their relationship, without talking about it directly. George would not take kindly if Dexter and Tracy started to rehash their marriage, but talking about the boat let's them skirt this.
It further allows Tracy to admit she wasn't very "yar" and for Dexter to admit his foibles. It also allows the establishment of the fact that Ms. Imbrie was at their honeymoon sailing taking pictures until Dexter tossed her camera into the ocean. This in turn provides reinforcement that Dexter is at heart a stand-up guy because he paid for the camera (and for the others he destroyed).
But I think the biggest reason for the boat is Tracy's reaction to Dexter considering its sale. She explodes and says won't stand for it. If we take it as a metaphor, she's says she doesn't believe in exploiting their relationship for profit, which echoes Dexter's sentiment "I had the strange notion that our honeymoon was our own". Yet, Tracy's fiance, George, shows himself willing to treat their marriage as an asset to be used as needed. This is another signal that they may not be destined to be together.