I know little to nothing about the development process behind the Little Mermaid ride we ended up getting. There is plenty of concept art floating around though. The figures don't look far off from what the art entailed, but the ride is a LOT darker and less colorful than the artwork. They also dropped the ball with making the sets look organic and detailed. Walls and ceilings are also rather barren and appear unfinished with exposed backstage objects.
I'll also mention Mermaid Lagoon land at Tokyo Disneysea. Because while there isn't a proper dark ride there (I think one was planned but scrapped), the interior complex is rather impressive in its own right. It shares a number of similar areas to the ride scenes (like Under the Sea, or Ariel's treasure grotto), but Tokyo's has a LOT more color and detail. Still some minor issues with the exposed ceiling, but even the surface waves are a lot more intricately detailed than the ride we got.
There was also a completely different Little Mermaid ride that was originally conceptualized back in 1992. It was designed by Tony Baxter and would have been built as an expansion to Disneyland Paris. No doubt canned due to that park's issues. There's a CGI ridethrough released as a bonus on one of the movie's DVD's. VERY ambitious, suspended dark ride ala Peter Pan but with vertically structured multilayered scenes. And a proper climax with a giant version of Ursula.
