Tod (Martin Milner) falls for a swimmer (Diane Baker) at Florida's famed mermaid tourist attraction.
Route 66 enters The Twilight Zone (1959) as, unlike the rest of the series, the episode is totally grounded in fantasy. When a TV show suddenly does a totally out-of-place hour, that hour sometimes becomes the most memorable episode of the series - that is the situation here.
People remember just one episode of Dynasty 2: The Colbys (1985), the final hour when the UFO landed. Because it was so different from other shows. People remember just one episode of Lost In Space (1965), the talking carrot episode - because it was just so amazingly stupid.
The Cruelest Sea of All is not just different, it is well made - with knockout location filming at a real Florida mermaid attraction.
Interesting that Diane Baker also appeared in the most memorable episode of sister series of the day, QM's The Fugitive (1963) - the final episode.
If you seek another 60s mermaid hour I would watch a third season Voyage to the Bottom of the Sea (1964) episode just titled - The Mermaid.