Okay I have no idea where this idea came from, but....
A mad scientist decides to try to turn his girlfriend into a mermaid. Or just some woman he grabs off of the street, lol. But while he develops the tail, and the drug to make her breathe through the scales instead of her nose and mouth, he begins by binding her legs tightly together and starting to teach her to swim by using her bound legs. But he has a slight twist on the mermaid, as she won't have the use of her hands either to help her swim.