The Spirit of the Deep

Darwin thinks he has seen a mermaid though it turns out to be an actress called Shemerlda (real name Doris) shooting a television commercial. Prof cautions drunken Cyril not to set out for Dunkirk in his leaky old houseboat 'Spirit of the Deep' full of livestock so when Cyril tries to put out to sea Prof brings Shemerlda on board to seduce him into stopping. And it works - though not quite in the way Prof had envisaged.