To sum up this story I would say for every good idea it had there was an equally bad one. Shakunetsu no Nirai Kanai was cancelled within a year of publication so that says a lot about the overall story. It's a miracle that the author was able to complete the tale with just 47 chapters. If you read the manga, the answer to the question why was it cancelled so early becomes fairly obvious.

Like I said for every good idea the story had there was a bad one. Let's start with the good ideas. Dolphin cop raising a little girl on a remote island that is home to a sea worshipping cult with magic ocean powers. That is definitely one of the more interesting settings and a premise with a lot of mystery and potential. You had strong characters with stronger personalities and stronger muscles. Even a unique fighting system and a lot of research into aquatic physics and biology. At the stories core, it was a shonen with a lot of potential that really nailed the shonen jump formula.

Of course then comes the bad and honestly borderline amateur ideas and concepts the story used. The biggest flaw comes with the hardboiled cop motif. Reading the story made me think that the author did not have a good grasp on detective genre or cop stories. Everything was resolved either too easily when it came to the actual police work or just became overbearing. The "I'm a cop and uphold the law" dialogue and mindset became a hinderance at times of high tension. The second biggest issue was the character of Chaoko, the young girl. Her entire power from the beginning was one of those ideas that was never properly fleshed out and made little sense no matter how many times it was explained. Honestly it was a bad idea from its inception and it was too late to change it. The third were the mermen species. This one was poor and unimaginative given that the merfolk were humans gangers that would sprout weird fish like appendages from strange places. Even the underwater setting was just modern Japan under water. There was nothing unique or creative about it. Zero thought went into the merman culture, city, or anything. There are several other minor things within the story like someone playing incorrect notes in a song. Nothing that disrupts the story but you notice. There were a lot of silly fourthwall breaking and dump jokes that struck as the work of someone who didn't know what to write so they gave in to the temptation of breaking the fourth wall.

Of course since the chapters of the series was limited and had to be finished so quickly, the ending, while complete, was rushed. The final fight was extremely quick and you can tell where the author knew it had end quickly. While I am glad the story had a ending, I was neither happy with the ending and honestly wanted to see the story continued to be fleshed out and continue on with all the characters.