Quite a few people have written in to tell me about this Australian television show, which aired from 2008 until 2010. At the series' start, it's about three regular teenaged girls named Cleo, Emma and Rikki who are given the power to be mermaids. The story them follows three girls' coming to terms with their newfound abilities, and the wacky hijinks that ensue to ensure they're not found out. The three girls also have their own individual powers: Cleo can manipulate water into shapes, Emma can freeze water and Rikki can boil water. At least, this is set-up for the first season... I haven't seen the rest but I've been told there are new mermaids who join up, power upgrades, and other changes in the stakes in the story.
This show has a few similarities with another Australian production (Mermaids) in that the light-hearted tone is about the same, and there are three mermaids who each have their own individual powers, but the mermaids in H2O: Just Add Water are all humans who are given the ability to become mermaids and the stories follow teenage problems and stakes. An interesting aspect to this show is showing how the girls have to cope with their powers both giving them advantages and (even more distinct) disadvantages in what they can or cannot do.
After this show ended, it was followed up with Mako Mermaids.