Pretty boilerplate stuff, as I recall, with big late-series reveals for just about every character. Niddler is reluctant to go to an island of monkeybirds they find, and we find he initially fled because of some bullies. Gets resolved in the usual way, and he's seen as a hero instead of a wimp due to his adventurous tales. Tula was/is a princess, and ducks an arranged marriage. Ioz, I think he gets shafted with no major reveals that I can recall.
Octopon, the super-advanced lost ancient city that gets partially revealed halfway through the series, is the root of Dark Water. Their lust for an energy source caused them to drill deep into the planet, releasing an ancient evil that slept there (which was suggested to be some kind of irradiated crude oil). The treasures are required to power a giant temple machine in Octopon that will suck the goop back into Mer's core. Like most cartoons of the time, it was a gigantic allegory for environmentalism, with sort of an "it was earth all along" post-apocalyptic vibe.