Octavia survived banishment to the crushing depths of the Trench by carving away the merman she was expected to be. She's managed to build a life for herself where she is genuinely happy and able to give back to those less fortunate than her. And what does she get for it? Deadnamed by her brother-in-law and blackmailed by her trashy niece.
She gives her niece legs in exchange for her voice so that the stupid girl can go to the surface and try to bag a prince. But the girl's beauty was a trick played by the distortion of water. She ended up a drooling mute with bad hair and trollop vibes.
So Octavia says goodbye to her hips and watches her own breasts fade as she stuffs herself into a man-shaped suit and heads to the surface to help the helpless. But the Prince is a Queen, so to speak, and he has other plans.