In 1991, Takahashi Rumiko's manga, Mermaid Forest, was adapted to a 1 episode OVA. In 2003, Mermaid's Forest expanded to an 11 episode TV series. I have not seen the original 1991 version, nor have I read the manga, but I am now tempted to find them both. Mermaid's Forest was one of the best series out in 2003, as it intertwined themes of fantasy, horror, and comedy with an engaging storyline and commendable storytelling presence.
It's actually a lot like the Highlander series, which aired in America sometime in the '90's. The main characters, Yuta and Mana, are immortals because they have eaten flesh from a mermaid. However, not everyone who eats mermaid's flesh benefits from immortality and can be ravaged by the disastrous side effects. In fact, it is quite rare, which makes Yuta and Mana special, in some biological sense. Basically, they travel together, relive memories, and partake in dangerous adventures, and so on, just like in Highlander. Additionally, contrary to most of Takahashi's works, Mermaid's Forest is fairly graphic and in many respects, morbid, as characters are always being killed, usually viciously, and others just die.
Technically speaking, Mermaid's Forest is much like other Takahashi works, with artwork resembling dated, but not ineffective, Ranma ½ style. In fact, I really enjoy this type of animation, contrary to most anime out now, which are all shiny and CG, which I suppose at times is appropriate. Hopefully, the type of artwork utilized in Mermaid's Forest does not deter viewers.
Again, the storytelling aspect of this series was superb. Never was I bored or disappointed by the actions of the characters or the turn of events.