The Pirates of Dark Water

Notes:
* Set on the planet of Mer, an ocean world consisting of 20 seas and various islands and other more fantastic locales. There's no mention of mainland or any significant landmasses greater than some small-ish islands capable of holding little more than one town or city (Octopon being the only mentioned city).
* Magic exists, though mostly in form of artefacts.
* Ecomancers are nature magic users of apparently some renown as people recognize them. Other forms of magic (besides Ecomancy) are not identified.
* Certain areas of science are surprisingly high, such as biotechnology (or this is another form of magic, as the practitioner called it biomancy)
* Instead of technology, native animals with their rather exotic capabilities are used instead (for example, there's an aquatic animal that functions as scuba gear).
* The Dark Water is tar or oil-like living substance created by the Dark Dweller at the center of the planet that devours everything it comes in contact with.