Nightmare Fuel / H₂O: Just Add Water

• Emma freezes Miriam once, almost killing her.
• The trance-like apathy the girls fall into when looking at the full moon and how it sets in for good once they touch water; the following day, they usually can't remember much of what they did the night before. Considering that the influence of the full moon borders on a heavily intoxicated state, just about anything is possible.
• The girls' Super-Power Meltdown whenever a full moon occurs. Rikki's first full moon is an extreme example — at first it's rapidly boiling up water around her, making bottles and foods explode, but when Emma can't even reach out to her without getting scalded, Rikki runs away to Mako Island, scared and confused...and proceeds to unvoluntarily set the island's forest on fire around her. Hell, when she kisses Zane, she comes close to killing him.
• Dr. Denman. That woman apparently has zero compunctions to kidnap four underage teenagers, blackmail three of them into complying with her wishes by holding the life of their friend over them, and plans to just keep them prisoners for an indeterminate time to experiment on them, threat of physical harm always included, while their families would never learn what happened to them.
• The mutation Emma goes through in a Season 2 episode after cutting her finger on a piece of coral. Her arm turns white and scaly and she starts to mutate into a monster.
• Season 2 episode "Moonwalker" shows a disturbing extreme of the mermaids' full moon insanity: Instead of giggly and loopy, they become eerily silent and hostile as they roam the forest on Mako Island, trying to drive away Charlotte who has pushed in on their camping trip and keeps following Cleo. They threaten Lewis to get out of their way, and when Rikki sees Charlotte hassling Cleo, she almost strikes her with a lightning bolt, terrifying her. She could have killed Charlotte...and Cleo just blankly glares at the latter and then walks away. It's an uncomfortable Foreshadowing to Charlotte's psychotic behavior during her full moon.
• The penultimate episode of Season 2 has Cleo being depressed, and she swims out into the middle of a known shark breeding ground. Whether or not she was trying to die is up for interpretation.
• The water tentacle that attacks the girls in Season 3.
  • In "The Awakening", it suddenly yanks Rikki off the dock. She starts screaming in an anguished tone which sounds different to the other episodes where she either raised her voice or yelled. The dramatic music in the background doesn't help either.
• The fact that if the girls' secret is found out, they'll end up experimented on. This comes close to happening in the Dr. Denman arc.

---