That documentary is a must-watch for any Miyazaki fan. They recorded the entire creation of Ponyo, from Miyazaki's watercolors to the final released movie.

It also shows why the movie ends as abruptly as it does and why Ghibli was eventually disbanded. With Takahata gone everything was bottlenecked by Miyazaki's himself, and he was simultaneously crumbling under the pressure and refusing to let anyone else have creative input in the studio.

For those interested, the documentary can be found in the video section of the Internet Archive.