Guinea Pig: Mermaid in the Manhole 1988

plot
An artist who copes with the grief of his wife leaving him by going into the sewer and painting the filthy and decrepit things he sees. On one such trip, he discovers a mermaid who has become trapped down there and brings her home to try and heal the festering sores on her stomach. Her condition rapidly grows worse, however, and as she becomes more and more disfigured by her illness, she begs the artist to paint her before she dies.