This series will likely turn everything you thought you knew about mermaids on its head. Just because it is about a mermaid does not mean this is a chick flick only women will enjoy.
It's a mix of time travel, action, historical drama, romantic comedy, crime mystery, and fantasy. It incorporates cross-cultural / inter-species / male-female conflict. The strong female lead Jun Ji‑hyun does a great job playing a member of a species out of her element, but also a kind of super-hero. A lot of comedic incidents ensue.
Never get enough of Lee Min Ho. Here he plays conman of questionable character and motives, who questions his own capacity for goodness, while the mermaid sees his potential to be better than he initially believes he can be.
Underlying this series are the questions, whether people can change, redefine their story, find redemption and give and receive forgiveness. But these are not questions posed in a heavy-handed manner. Rather, they are woven into the fabric of the story-telling, and creep into the viewers' consciousness unbidden.
Very enjoyable and worthwhile Kdrama.