I think it’s about the guy being unable to speak his mind, which hinders him from growing. On the verge of a faultless breakup, it’s all bubbling up to the surface.
He’s passive and stuck (like the reptile that stands still or crab that only moved when disturbed). when they’re together, she pulls him forward, but that’s ending with the breakup. During their breakup talk, she tries to get him to express himself more, poking him. She knows it’s painful for both of them, but she’s worried he won’t move forward when they’re apart.
During their last hurrah, he can see how she’s moving forward with her life (fish swimming) and admires her, but also how helpless he feels struggling to keep up.
She tries to encourage him to speak his thoughts just like he did after he was caught up the movie, and Bang Bang is the gesture/sound of him needing to speak up, for himself.
He finally shares his thoughts because he can’t take it anymore, and with his confession his pain comes out. He continues to be overwhelmed by the thought of him trying it (flying fish engulfing him) and she (and the concept of moving forward) feels unreachable.
She’s thankful he could tell her but continues to worry because he doesn’t come to terms with his feelings, trying to avoid pain.
At the airport he suddenly feels alone, and bang bang, the hostess asks him something and he has no choice but to speak for himself which shocks him.
A lone fish jumps…he’s done it.
She decides it’s best for him if he doesn’t have a lifeline and deletes his contact info, so she won’t be tempted to check on him. It’s all up to him to keep swimming, for his own sake. She points up to him to encourage him one last time, Bang!