The Pirates of Dark Water (also known as The Pirates of Darkwater, or simply Dark Water) is a fantasy cartoon created by David Kirschner that was put out in the early 1990s by Hanna-Barbera. It details the quest of a small band of allies from the alien world of Mer, who have set out to recover the lost and mystical Thirteen Treasures of Rule, in order to stop the Dark Water, a tarlike, semi-sentient substance poisoning Mer's seas that swallows anything it touches. The Treasures are the only things that have the power to push the Dark Water back into the core of Mer, where it's coming from.
This group of heroes consists of Ren, the prince of the once marvelous but now dying kingdom of Octopon; Niddler, a monkey-bird; Ioz, an ex-pirate who'll never turn down a chance to get his hands on gold or treasure; and Tula, an independent girl and an ecomancer from the island of Andorus.
However, Ren and his friends aren't the only ones after the Treasure; the feared pirate lord Bloth, whose hate of Ren's father, the late king Primus of Octopon, has spread to Ren as well, tries to obtain the Treasures for his own personal gain and to let him control the Dark Water.
What makes the show so unique is that it's one of the few attempts at a serious fantasy animation saga (not just a show, but a continuing storyline with a serious plot) by an American animation studio. It's actually been referred to on occasion as the Star Wars of fantasy and animation. The show began in February 1991 as a 5-episode continuing miniseries titled "Dark Water." The following year, it was rereleased (with some changes--a new voice actor for Niddler, some scenes reanimated, some scenes cut and others included, etc.) along with 8 more episodes, and the series was renamed "The Pirates of Dark Water."
Due to the studio hiring special (and more costly) animators for this series, and also because of a lack of popularity/ratings due in part to people being a little thrown by the then-novel idea of a darker and more serious animated series, ABC initially passed on renewing the story, and The Pirates of Dark Water was slated to end there, midway through the storyline. However, the creator and producer of the show (David Kirschner and Cos Anzilotti, respectively) pushed for continuity, and finally, 8 more were produced the following year, 1993, and shown during the Funtastic World of Hanna-Barbera cartoon block, with the Young Robin Hood series.
Around that time, Turner acquired Hanna-Barbera, and the executives in charge of the animation department sought to push their own projects forward, shoving the HB projects in development aside. As a result, the series ended after 21 episodes, during which the crew had collected 8 of the 13 Treasures throughout the course of these 21 episodes. The story remains unfinished, but contrary to popular belief, there were no further episodes that were developed or produced.
(Thanks to Scott, who worked for Hanna-Barbera's Marketing and Licensing Department, for clarifying the situation.)