This series ran for 3 seasons. My 9 year old and I loved the show, but it did include increasing amounts of dating, worrying about dating, arguing about boys, and general drama as the series progressed.
As the series begin, there are three nice young girls (teens) who unexpectedly turn into mermaids. They wrestle with issues relating to this and find their way as they straddle the land/sea worlds they now inhabit. For us, the show remained interesting through season 2, but we tuned out after watching the first episode of season 3. This was because of the increasing focus on dating and romance that started in season 2.
The show became very much like a sanitized teen soap opera. Someone was always arguing with someone over something minor, characters flip flopped between good and bad behavior and the girls focused increasingly on boys, boys, boys.
Still, the girls are still relatively strong and interesting characters, as is their friend Lewis, although the character development is uneven. Some of the traits presented originally seem to fade away as the season goes on. Emma, for example, is from a tight family but seems to grow farther and farther from them without any clear reason. This was disappointing to us - we wanted her to share her secret with her family, instead of lying to them. In the early episodes, lying to her family was a huge problem for her, by season 2, it was not an issue at all.
In general, as this show progresses, the girls become more focused on their looks (more makeup, less clothing) and boys (more relationship drama) and less on the things that used to matter to them (family, school, friendship, honesty).
One nice thing however is that Charlotte (introduced in season 2) is refreshingly healthy in size for a TV actress. She is less slim than the other girls and in a healthy athletic way. On the flip side, Cleo gets very thin in this season, near skeletal - this disturbed us - she really looked ill; but Charlotte is a nice counter to this. (In season 1, all three girls are relatively healthy and natural. The main hang out for the show is a fresh juice shop.) It was a shame the writers and producers took this show from what could have been a really healthy show about healthy girls and turned it into a run of the mill teen soap opera. It was fun to watch for a while but, for us, it lost its appeal.
We can, however, highly recommend the spin off - Mako Mermaids! See my review for that - it's a much better choice for young viewers!