Talk Dirty to Me 4 (1986)

The hot and horny mermaid from "Talk Dirty To Me, Part Three" returns... this time bringing her sister mermaids to sample the sins of the big city. Unfortunately, the city water becomes poisonous to them and they must return home. Alas, our aquatic heroine is nowhere to be found. Her quest for sexual gratification has led her astray. The mermaids seek out Jack, her original tutor in titillation, since he knows what mermaids like and has plenty of bait. Jack agrees to find her and discovers once again that what smells like fish, and tastes like fish, may sometimes be a fish.