You give your rival information that will go beyond what you normally say and you realize the two of you are in the same boat on a lot of personal things. You are bitter enemies in the same boat! Or classic rivals, like you see in movies and read about. You never would have known if you hadn't gone beyond what you normally give people.

This is a jukugo word that uses the on'yomi readings of the kanji. You haven't learned this reading for 舟 yet, so here's a mnemonic to help you:

What are bitter enemies in the same boat supposed to do? They shoot (しゅう) each other!