These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: H2O: Just Add Water
Angst? What Angst?: Cleo and Kim's parents divorce offscreen between season one and two. It's briefly mentioned, and neither the girls nor their father show any emotional turmoil at all.
Base Breaker: Emma. She's either a well-meaning girl with a few neurotic tendencies or a bossy and controlling harpy who tries to lecture her friends every chance she gets.
Designated Hero: Quite frankly, all of the main characters from time to time, who continually behave in petty and selfish ways and never really learn from it or properly outgrow it. Some outstanding examples:
After Cleo gets fired from her job and Lewis takes the position, Rikki and Emma sabotage Lewis at his job in the hopes of getting him fired so Cleo can get her job back, which leads to Lewis being the prime suspect in a police investigation.
Despite knowing that Rikki is poor and lives in a trailer park, Emma and Cleo (who live comfy middle class lives) get angry with Rikki when they learn that she's trying to find something that has a reward attached to it.
Rikki, Emma, and Cleo sabotage a family dinner between Cleo and Charlotte's father in an attempt to drive Lewis and Charlotte apart - never mind that at this point, Lewis was single because Cleo turned him down, and therefore fair game for Charlotte, who at this point has been nothing but a decent person.
After Charlotte's initial encounter with the moon pool, Lewis deliberately gets her wet to see if she'd gotten mermaid powers. Here's the kicker: they were in public at Juicenet. Not only would this have broken the Masquerade, but Lewis would have just exposed an innocent and confused teenage girl to the public if she had gotten mermaid powers. Neither he nor the girls seem to see any problem with this.
Designated Villain: Charlotte, who before going all-out evil doesn't really do anything wrong. Can you really blame her for getting a tad upset and suspicious when Lewis continually lies to her, or runs off or stands her up with little to no explanation to go and hang out with three other girls?
Fridge Logic: So how do they avoid being seen in the school loo? Do they just not go in public?
I'm not sure why that would be problematic for them. People don't normally get splashed with liquid when going to the bathroom, if they're doing it right.
Yeah, but do they just use Purell or what?
Brought up in-series when Kim notices she's never seen Cleo washing her hands.
They have ten seconds to dry off before the change kicks in. That's enough time if you're quick, have paper towels handy, and maybe wash them twice.
"The Siren Effect" - Cleo says it's a Wednesday. So why is she having a sleepover on a school night? And why aren't the girls getting ready for school the next day?
Germans Love David Hasselhoff: It's a hit in The UK, Central and Eastern Europe, to the point that the 3rd season aired in the UK first before Australia got it... by a year.
Doing rather comfortably in the U.S., too.
Moral Event Horizon: Charlotte crossed it when she in the second-to-last episode stole Cleo's lockett and made her believe that Lewis no longer cared about her.
The Scrappy: Good God, Kim. Many, many of the show's fans have expressed the deepest desire to punt her through a window for her constant attitude.
Take That, Scrappy!: She does get plenty of slapsticky comeuppance though. She gets publicly embarrassed when she tries to out the girls' secret, catches the measles, gets sprayed with a garden sprinkler and is dumped by Eliot for being too bossy.
Toy Ship: One episode had Emma's younger brother Elliot trying to ask Cleo's younger sister Kim out. Kim just took advantage of him and kept bossing him around. He eventually got tired of her attitude and ditched her at the end of the episode.