@SGR:
The focal characters are Fisher Tiger and Otohime, and the focal point is the hate between Humans and Fish[er]men.
I agree and disagree. The hate between humans and fishmen had been told and retold through Arlong and Shabody Park.
To me, this was a story of three paths. Otohime's path of forgiveness, Arlong's path of revenge, and Fisher Tiger's ill-fated middle path. The flashback showed how each character chose their path and laid the foundation for the final clash that's about to come. It's okay to feel unsatisfied with the flashback, though – I don't think the flashback's full significance has been realized yet. It still has shots left in the clip, and I trust in Oda to fire them at the right times.