It`s about three girls: Emma, Cleo and Rikki (Emma is replaced by the new girl Bella in season 3) who magically get turned into mermaids and are each granted a power over water, which they each must learn to master. (Emma can freeze water, Cleo can shape it and Rikki can heat/boil water. Their powers grow and expand to other things in season 2, such as Rikki getting power over fire)
Whenever the girls come in contact with water, they turn into mermaids, and they have difficulties hiding their secret from their families and friends while keeping up with their normal lives at school, work, etc.

I know it might sound a bit lame, but this series is actually quite enjoyable. It`s primarily for teens, but I`m not far from turning 27, and I recently finished watching the whole series and found it to be quite entertaining. It`s a take on everyday teen problems with a touch of magic. If you`ve seen Aquamarine, you`ll most likely love this.