Casting Gag: The Spanish voice actresses for Karen and Coco already played mermaids in H2O: Just Add Water a year earlier. Similarly, Mikaru Amagi's voice actress played a H2O character whose role in the plot was similar to Mikaru's.
Cross-Dressing Voices: Hippo, Michel, Nagisa, and Lady Bat are all played by females in the original Japanese version.
Jossed: The Great One is not Michal. Or Aqua Regina.
No Budget: The anime. The animation's often Off-Model, there are a lot of Stock Footage and flashbacks alike, some of the voice actors couldn't actually sing (though most were pretty good), and the animators didn't even redraw the lip flaps when reusing stock footage for different songs. The last episode of the first season has a glaring example where all they did to spice up the final battle was add rainbow effects over everything and occasionally obviously speed up some clips.
No Export for You: Averted: we've got the manga in North America, and other countries have it, the anime and sometimes the licensed games. The reason the English-speaking world hasn't got the anime is because TV wouldn't take it, not because of lack of trying on either side of the anime industry.
What Could Have Been: It was to be set in the pirate days, with Kaito as a pirate. The only heroic mermaids were Lucia (then named Lyre), Hanon and Rina. The villains were also merpeople and were only Izuuru, Gackt/Gaito and Sara.