The Splash Effect is the rule that nearly all mermaid shows follow: when a human-form mermaid gets wet, she pops a tail. When she dries off, she gets her legs back.
This is actually not something H2O made up: it comes from the film Splash (of course), made in the 1980's, though H2O is probably the reason most YouTube shows follow it. That's also where the ten-second rule comes from, probably for dramatic purposes--long enough to hide, short enough for tension.
When it comes to storytelling, I'm not a fan of doing the same thing everyone else is doing, just because everyone else is doing it. Also, the Splash Effect specifically tends to result in contrived wettings. Suddenly everyone is drinking water and spilling it willy-nilly, having splash wars indoors (seriously, when was the last time your parents let you do that?), and so on. Worst of all, the mermaids get dumber: either they forget that this is going to happen and get themselves wet, or they completely spaz out at the mere sight of water, which isn't doing their secret any favors.
This is actually the more traditional mythological method: the mermaid has to put on her tail, like a selkie or a swan maiden, and possibly stores it in some object, like a seashell or one of those lockets mermaid shows are so fond of, when she's not using it. Using this method, you probably wouldn't need to worry about jump cuts.
The mermaid has to concentrate in order to gain or remove her tail. This could have a lot of the same drama as the Splash Effect, since one distraction could make her pop or lose her tail, but the mermaid would also improve over time.
When a mermaid's tail comes and goes according to some recurring event she has no control over, like the phase and position of the moon, the tides, etc. Consistency is the thing here: the mermaids would turn at a regular time, for better and for worse.
Not the one-time-use spell most mermaids use to trigger their initial transformation: a spell that the mermaid has to recite (a potion she has to drink, a gesture she has to make) every time she wants to switch forms.
There's nothing wrong with the Splash Effect itself (and incidentally, I think "The Splash Effect" would make a pretty good title for a show that obeyed it), it's how it's used. Still, I'd like to see some of these other methods used, just to shake things up.